Does a Glock 17 hold 17rds? I think you can get +2 extensions to make it 19rds. If I were looking for the most 9mms in a gun, that is probably what I would go with. That is until they make a XD-9 mag that holds 17-20 rds.
My Beretta 92FS holds 20+1 and that is with FACTORY mags not the Pro-Mag BS that you see around most of the time. I keep that for HD and it makes me feel good. I think 15+1 is enough most of the time but in a HD gun, I don't have to conceal it so the extra bit of mag sticking out th ebutt doesn't effect anything.
For a carry gun, I want compactness more than firepower. It is nice to have lots of shots but if it is such a big and heavy gun that you won't carry it, what is the point? 7 rds of 9mm is what I normally carry and some of the time 5 rds of .38spl is all that I carry.