It's really slow here at work, so I'll take a few moments to comment on the P-38 and P-51, and their comparative performances. We'll start with the ETO, since thats where the controversy over the P-38's performance comes from.
First off, a few notes about strategic bombing. The whole point of the 8th AF's strategic bombing offensive was to destroy the Luftwaffe. Their targets were airplane factories, and other facilities that helped the production of aircraft(particularly fighters). At the beginning of 1944, when the allies began looking to invade the continent, it was obvious that bombing the German aircraft industry out of existance to destroy the Luftwffe wasn't going to work. The 8th's bombers had been mauled a number of times, and the Luftwaffe in the west seemed as strong as ever. At this point, a change in leadership of the 8th lead to a new strategy, made possible by longer ranged escort fighters. The bombers would continue to fly to targets that the Luftwaffe would always defend(like the aircraft factories), but the fighters(which could now accompany the bombers to their targets) would now concentrate on shooting down fighters instead of protecting the bombers. The idea was to destroy the luftwaffe, so that they couldn't contest allied air superiority when the invasion came.
From the start, the Luftwaffe heavily contested every 8th AF raid into german air space. This was basically the case up until the Berlin raid. The day after the first Berlin raid (March 6 1944 i think) the 8th AF went back to Berlin and were virtually unopposed. At this point, air superiority over Germany fell into allied hands, and they kept it for the rest of the war. Now the Luftwaffe didn't just give up as the result of that one raid, but as the result of steady preassure that had been mounting since december of 1943, climaxing in "Big Week" and then the Berlin raid in early March. Thus we can identify this period (basically February through early March 1944) as the critical turning point in the daytime air battle over Germany. In the past, the 8th AF couldn't run more the a few deep penetrations in quick succesion before losses caused them to stand down, to allow the airmen to rest and refit. Now the situation was reversed. Loss of planes wasn't the problem, loss of pilots was. The planes could be pumped out at a faster rate, but there was no easy way to repair the huge holes in the luftwaffe's ranks, now that the allied fighters were flying over the entirety of Germany. In particular losses to the 110s and Ju88s that had been devastatingly effectivly in the bomber destroyer role kept the Luftwaffe from inflicting the type of devastating losses that they had on the 8th AF bombers in 1943.
Now if we look at the composition of the 8thAF fighter command at the beggining of Feb 1944, they had about 9 P-47 groups, 2 P-38 groups, and 2 P-51 groups. The two P-38 groups(55th and 20th) had both been active since the beginning of 1944, and neither had had much success so far. Of the P-51 groups, the 354th (officially 9th AF, but operating under 8th AF control) had been operating with some sucess since Dec 43, and the 357th had started up just at the beginning of Feb 44. Both the P-38s and the P-51s were having teething trouble. The P-38s suffered a lot of engine failures, and had compression and roll rate problems, along with defrosters, heating etc. The P-51s were suffering a lot of gun jams(some field mods were in place on some aircraft to fix this, but it continued to be a problem until the P-51D), and also some defrosting and spark plug problems.
Now over the course of "Big Week" the P-51 groups, one of them brand new, making up about 15% of the allied fighter total, accounted from about 27% of the allied fighter claims. And this is the 4 gun P-51B, with gun jamming issues, and poor rear visiblity, piloted in many cases by pilots with only a handful of missions in theater. Even before the beginning of "Big Week" it was obvious to the 8th AF fighter planners that they had found the fighter that they needed to win the day war over Germany. To argue that the P-38s were fazed out because of mechanical problems alone is unrealistic, considering that they didn't begin converting to the P-51 until late summer and fall of 1944, long after the P-38 groups had converted to the later J models which solved many of the problems. Also the 8th AF activated two additional P-38 fighter groups after this point, so obviously the mechanical problems with the P-38 weren't such that it wasn't worthwhile to operate them.
What's especially interesting is this: By the end of the war in Europe the 9th AF had converted all their P-38 groups to P-47s and P-51s. Now the 9th AF was doing primarily low level air superiority and tactical bombing, and so their P-38s wouldn't encounter compressibility, wouldn't have problems with defrosting or cold temperatures. And their P-38 groups also got the later J and L models.
We can see a couple important points here: First of all, the 8th AF was primarily interested in killing the Luftwaffe, and they found the P-51 was the best tool for this job. Obviously the P-38 wasn't the total failure that some might make it out to be in the ETO ... both the 8th and the 9th far preferred the idea of having some P-38s to having no fighters. Both the 8th and the 9th AF got to use the later J and L models, and they still converted all of them to P-51Ds when they had the oppurtunity. This leads me to say that all of the US fighters were dangerous, credible foes for the Luftwaffe, just some were more effective then others.
Now in the pacific, we have a very different situation. First off the primary fighters types for the IJN/IJAAF (the A6M, Ki43, and Ki61) are all much slower then nearly every allied fighter in action. Every wartime allied fighter(fighters that were brought into service after the beginning of the war, ie not F2A, F4F, P39, P40) was VERY succesful in this theater, due to their massive advantages in performance and durability.
Interestingly, the P-51 was the least sucessful of the US late war fighters in this area for a couple reasons. First it had very limited exposure to combat, due to the huge demand for P-51s in Europe. Some were used to good effect in the CBI, and they didn't get into action in the Pacific really until the fall of Iwo Jima. At that point they were used to fly escort missions for B-29s against the Japanese home islands. However it was soon discovered that the high altitude bombing wasn't producing the kind of damage that was hoped for. In this case, the AF commanders were looking to wipe out Japanese industry and convince japan to surrender, and weren't interested in the IJN/IJAAF(both of which had been smashed to almost ineffectiveness over the course of the war). It was found that low altitude bombing at night was more effective, and so the P-51s were left to conduct fighter sweeps and airfield attacks against the home islands. Due to the very fast jet streams, long overwater approach, and unpredictable weather, a B-29 still had to accompany the Mustang formations for navagation. In the end, losses to weather, exhaustion etc. were far greater then the kills that the P-51s could inflict, as there was hardly any enemy left to inflict them on.
Also intersting, is that once again P-38's were almost completely phased out by wars end in this theater as well.
Anyway, I would judge the P-51 to be the better overall fighter(air to air) for a couple reasons. First, its faster at almost all altitudes. Second it was regarded as a very good handling aircarft especially at high speeds. There were exceptions to this, primarily low speed turning situations with a full fueselage tank, but the saying goes "speed is life", and as the RAF noted in 1941 when the Fw190 came out "turning doesn't win air battles". Thirdly at the Navy Joint Fighter conference in october of 1944 the pilots present ranked the planes in a number of catagories two being "Best overall fighter above 25k" and "Best below 25k", and the P-51 was one of the top planes in both of these catagories(quite an accomplishment saying that the vast majority of the votes came from Navy/Marine pilots and contractors from Vought and Grumman). Also the P-51D had much better visibility then the P-38 too.

Oh well, enough for today.
Sable
352nd FG