If you want it as a backup for hunting, I would go with a larger caliber like a .45 or 10mm. I only stay 10mm because I have heard that it is one of the best calibers for a hunting backup (not really sure why) but the ammo can also be rather hard to find and somewhat expensive if you plan on shooting a lot. If you DO want a Glock, they have pretty much every caliber you would want available. I have a Glock myself, and I love it, and I have heard a lot of good things about Sig Sauers, but I've also heard they have a very strange trigger pull which some people like, some don't. I would recommend shooting some of each before you buy one.
Overall, if your main purpose for it is a hunting backup, I would recommend a .45 1911, but it also depends on what you're hunting... You wouldn't want the same gun for hunting rabbits as you would for hunting bears.
Also, you're not going to want a compact handgun like a Glock 19/23 for hunting...
Check ammo supplies around you before you buy. Here in SE Michigan, you can't find anything other than .40 in stock anywhere without looking pretty damn hard.