Urchin,
I disagree and think the 50's are better. Unless you are a very good shot with the La7 you are going to have serious problems with any sort of tough shot, or one at range. The La7's cannons just fire all over the place unless you are almost point blank. Point blank I agree the 20mm's are better, but under any other condition the 50's are superior. Volume of fire, accuracy of fire, range, etc... all favor the 50. Even in a snapshot landing 10 50's hits is often better than 1-2 20mm. More criticals. Sure, it isn't always instant death like a point blank 20mm broadside, but the writing is on the wall when those 50's start slamming home.
Round for round the 20mm's are better when they hit, but that's only part of the puzzle.
-Soda
The Assassins.