I've heard/read some questions regarding the 357sig. Many feel that is a good round, but point out that the 9mm +P+ is pretty darn close to it. 40s&w advocates think that the energy gain is from velocity and think that over penetration is likely. Others think that soft cover penetration make it a better round than either 9mm +P+ or 40s&w.
If it were my only gun I'd opt for the 9mm as the ammo cost on 357sig is higher for range time. 9mm gets you more capacity with very close balistics in the +P+ loading. Possibly the 40s&w model if It was to be the only gun, knowing that with glocks Barrel/spring/mag changes would allow to go 357sig or 9mm later. If your looking to get all the calibers eventually, and like the Idea of the 357sig, go for it...
Personally since your going full size you might also consider the 45ACP model 21. (losing two rounds cap, easy to find ammo and better variety) Or the Model 20 in 10mm if power is your primary concern, and recoil is not an issue for you (or ammo cost).
hope that confuses things for you.