It is really a matter of preference, Gabber.
The pros and cons of the two armament types are so clear that the only way to decide which is better is according to personal taste.
Generally, Bf109s have a hard time gaining position to fire at, and you don't really have much time to aim. Now, couple this with the fact that the MK108 has low firing rate and bad trajectory, hitting a target with that gun can be a real pain in the arse. Therefore, some prefer the MG151/20 - lot better speed and trajectory.
However, considering the hectic nature of the MA, fighting too long against 1 enemy is very dangerous. Besides, even though the MG151/20 is significantly easier to hit than the MK108, compared to Hispanos or .50 rounds, it is still an inferior weapon. Thus, using the MG151/20 actually may not guarantee a better chance of hitting the enemy. Thus, you can look for just one, just one little chance to land a hit, and you can expect the enemy fighter to go down in flames if you are using the MK108. So, some prefer the MK108.
In my case, for fighting in a mildly dangerous area, where both friendly and enemy numbers are low and equal, I prefer the MG151/20. Lot of 1 vs 1 fights in these areas, and even the slightest different in hitting performance counts very much in 1 vs 1 fights. You don't really need a 30mm for this.
However, when I see enemies begin to swarm to a field, but the defenders have some time to prepare for a defensive, I use the R6 package - using 20mm wing cannons and 20mm spinner cannon. They degrade overall performance, but since there is enough time to get ready, and there are many fighters you must face, 400 rounds of 20mms really help.
In a really dangerous situation, I use the single 30mm cannon. The chances are, you are not going to stick around long in dangerous situations, and gonna have to fight with very keen SA, prompt executions and withdrawals - something to bring enemies down in the shortest amount of time.
For buff hunting, the R6U8 package, with 20mm wing cannons and single 30mm cannon. This one's self explanatory
