My primary recommendation would be ,that for the time being, to stay with the 3 most common calibers (9mm, .40, .45 acp). It is usually best to stick with the ammo selections that you are most likely to be able to find. This would be true for other gun makes as well.
There are probably enough 10mm and .357sig pistols out there, that you'll be able to find some ammo for them. There won't be as many choices in ammo and it will be more expensive. Also holds true for all gun makes.
The 40, may be a little snappier to shoot for some that aren't familiar with it. The 45 will carry a few less rounds, but probably carries enough to get the job done. Te 9mm will have the least expensive ammo on the market. It's really a preference thing.
Many of the midsize Glocks do the job of a full-size pretty well. In addition to a full 22, I shoot a 23 and 19 and actually prefer them.