If your specifically looking for a 1911 style weapon then I would go with a GBB. There are many reasons for this but the main one is(in my opinion)the NBB 1911's don't even come close to the GBB 1911's.
Really the pros cons between GBB's and NBB's are pretty even. A couple things the previous posts didn't mention is:
-GBB's have much more replacement parts vs NBB's(even the TM Mk23 has a lack of parts).
-GBB's also have an intimidation factor over NBB's.
-GBB's are much more fun to shoot.
-Field stripping is easier (usually) on a GBB.
-very few manufacturers make NBB inner barrels, however FF and Nine ball have a bunch barrels for GBB's.
-GBB guns have a much shorter trigger pull
-the gas report on a GBB is distributed evenly, whereas all the gas comes out of the front of the barrel on a NBB(this could be a pro or a con for a NBB)
On the flip side:
-NBB's are very quiet compared to GBB's
-As mentioned before NBB's are very efficent compared to a GBB.
-NBB's usually can have working silencers(basically a smaller AEG silencer with Foam in it).
-NBB's have less moving parts
-NBB's have alot less small springs(which depending how you treat your GBB brake easy)
If you get a 1911 check out the TM,
http://www.airsoftsmith.com/store/product_info.php?cPath=21_23&products_id=755By far the best 1911 i have held, I know its not all metal but all that means is you can use it more effectively in the winter.
Not to bash Doubletaps recommendation but it seems that WE guns like to fall apart after about a year or so. But if you like to flip guns alot(buying a gun, maybe doing a mod or two then selling it this would not be a problem).
Ive heard from all the users of WA pistols that they are by far the best 1911's, I personally haven't used one because they are so rare; but I'm sure you will not be disappointed.
http://redwolfairsoft.com/redwolf/airsoft/Category?menu1=2The TM Mk23 is by far the best pistol ive ever shot(performance wise, I think the Deagle kicks its but in the fun category). If you watch all the PhantomTMA vids on youtube youll see what I mean; they get a good 120ft with em using .30 Gram bb's!
Linkage:
http://redwolfairsoft.com/redwolf/airsoft/ProductDetail?prodID=9224Ok I have wrighten my necessary page of info:lol I'm done. If anyone has any experience with some of the Co2 1911's I want to hear what you think of them. Not that no one but Co2 users should post but I'm a little knowledge lacking in that area.
sticks
Subject: Gas Pistols, Non-Blowback or Blowback 
Wed Sep 03, 2008 7:22 pm
Also, another thing I forgot to mention, which is wierd, because it's the main reason I prefer GBBs over NBBs, is because often, from what I've seen, NBBs are either very difficult to take aprt, or impossible to take aprt. GBBs on the other hand, almost all have very easy dissasembly, (I can take apart my Luger and my .45 and put them back together, blindfolded: NO JOKE.) which is a very important asset because lets face it, those little crazy springs and pieces in the the gun, no matter how high-quality, are gunna wear-down or break sometime. and when it happens, you need to be able to access it. If I wouldn't have been able to take apart my .45, I never would have been able to replace the Recoil guide, the spring, or fix/get rid of/clean out either of the two BBs that exploded inside the gun. (They were quality BBs, too)
