If you like the Glock - there is nothing wrong with it. I've shot them at my range - but I don't own one - or any Glocks for that matter. My biggest complaint with the Glock is the trigger mechanism - play, the way they reset, etc. It just isn't to my taste.
Personally, I would recommend one of the Sig 226 models instead if you really want something in that caliber.
I haven't handled the G31 but I have fired a G17. I didn't like the trigger safety mechanism or the cheap (for lack of a better term) overall feel of the gun. It felt more like a toy than a weapon. I have shot a Sig P226 and I really like that gun. It has a solid, quality feel to it and I believe the .40 can be converted to .357 and vice versa on several Sig models with a simple barrel switch.
The 9mm Glock was fairly jumpy due to the light weight frame. I imagine the .357 would magnify the muzzle flip. Some people love Glocks, I love revolvers, but when it comes to higher priced semiautos I'll go Sig if I have the option.
Try to rent one of each if you have the option. You know what they say about opinions....
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).
When you get into any calibers other than 9mm - unless you reload - the ammunition gets pretty expensive. If you like reloading, and I do, going with the .357 sig caliber is not really an issue then.
But I agree with the others - I wouldn't look at a .357 sig as a first gun probably. But if you like the Glock - go with it. But like one other responder suggested - try and borrow a buddy's gun or go to a range with some rental guns - and shoot their rentals before you buy anything.