Carbon steel has a slight advantage over stainless if you are building (machining) a 1911's slide/frame. It is slightly "softer" and easier to machine. Carbon steel pistols tend to be less expensive than stainless, partly because of this reason.
Stainless has better rust resistancy, and is easy to refinish if you should ever scratch it. Carbon steel with blued finish, would require a complete re-bluing if you were to repair a scratch - although cold blue can look OK - although I have not really found one that is durable enough.
I agree with Shane, I would also add that many shooters feel a carbon steel gun seems to cycle slightly smoother due perhaps to different coefficient of frictions in Carbon and Stainless. Some don't notice it, some do. On stainless guns, you sometimes need to worry about galling (pitting) of the material which is essentialy its mode of rusting, but to a less dramatic extent. Both are great materials and both require maintainace. If you have any hope that you can at least wipe down a gun for 2 mintues after you shoot it, I wouldn't fret to much.
Galling has nothing to do with rust or corrosion. When two stainless surfaces are in sliding contact, there is the possibility that they will "cold weld" and sieze up. If forced, the surfaces can rip gouges from each other.
I'm no metallurgist, but I understand this phenomenon arises from nickel content, and is more probable when the two surfaces are made from the same stainless alloy.