Having owned all of them, sometimes several of each type, my personal preference is in a pistol platform to stay with things that are of a common caliber to other weapons you already have.
for instance if you have other 9mm handguns, i would likely recommend you stay with 9mm and make your ammo restocking operations easier. the same would go for any other caliber as well.
i just sold my Five Seven USG on friday. although i'll miss it (i also sold 11 other weapons on Friday) the most, unless i was going to also obtain a PS90 or get a tax stamp and go through the waiting period and get a P90, it really didn't make sense to own it. it's an expensive cartridge to get into with SS197SR regularly running $.60/round. it's also non-reloadable. i can get 3x .45ACP rounds or 3x 9mm rounds for that price and when it comes down to it, practice makes you better, not higher velocity rounds for a special purpose platform.
sheer cool factor? five seven
classic cool? 1911
rough and tough and don't have to worry about it ? glock
more weight and money but also as reliable? sig
i don't ever see myself getting into .357Sig, .45GAP or those other special purpose or high pressure rounds. 9, .40 and .45 should get it done. if not those then .357Mag and if i need bigger then that then i'm going for a rifle (and yes, i've owned .44Mag, .454Casull, 5.7x28, 10mm, .50AE, 7.65x25, 9x18Mak, .380, .25ACP, .32S&W-L, .32ACP, .35ACP and a bunch of others).
just my $.02
maybe buy a glock 23 and just get conversion barrels?