Actually I find it's better to get kills with the 30mm because with 200x20mm rds you have the possibility to get... maybe.. say 3 kills.
Okay okay *I* might have gotten 3 kills. 4 if I got a lucky kill in there somewhere that took one burst.
With the 30mm it takes MUCH more work to get that perfect kill (fly right up their butts and let 'em have it! - type of thing) but you still have plenty of ammo left over after the kill. So if you had the gas and kept going you could tally up maybe 6+ kills (I got 2 kills in a row with only the 13mms on a G6 once).
So it makes it harder but when you DO get kills you usually can make the ammo go longer than 20mms.
It's just a feeling type of thing, no actual evidence or proof.
Oh and 30mm kick arse for blasting B24s outta the sky, too!
EDIT: That's assuming you don't take gondolas, which you can add anyways and get 250rds 20mm on top of your 60rds 30mm
