57mm + 76mm would probably be better than two of either, but commonality is a big deal on a small ship, both for spare parts and for trained maintenance/repair personnel.
Hmm, I'm kinda surprised. I'd always thought that if you had one, that's all you need - one, since both guns have a very fast rate of fire. And unless you really do expect to go up against big ships, one will do as well as the other.
Actually, personally I'd rather see a Millennium Gun right where the 57 is, just because I have doubts about where that 57 is and I know the Millennium Gun has minimal deck penetration.