I don't carry, but I plan to as soon as I leave Maryland, so while I have no experience carrying I have practiced holstering and drawing my guns often so I'll be ready.
That said my preference would be both, depending upon the mission.
Generally, I prefer a revovler with a hammer. I think it looks better, it allows single action shooting when you want it (mainly at the range or a home defense situation), in an IWB holster it shouldn't catch on clothing (I've practiced enough, often with my winter coat or suit, to know) and you have more choices in guns.
I don't worry about the single action option for CC purposes. In a defensive shooting situation most people can shoot double action much faster than they can cock the gun and shoot single action. The distances are short enough that the accuracy gain in SA doesn't matter much. Also, with enough DA practice most people can get good enough with DA shooting to rely on it. In a home defense situation, however, I'd want the hammer because there may be enough warning to benefit from it (though I'd prefer my shotgun anyday
).
With that said I certainly would prefer a "hammerless" design for a gun that was to be kept in my pocket. That is when snagging can become a problem (often it may not, but in a life or death situation why risk it). With a good holster lint and dirt probably won't get in the gun if it has an exposed hammer, but again, it may not happen often but I wouldn't want to risk it.
I don't like the concealed hammer over the hammerless because it seems, like many compromises, to offer the worst of both worlds. Like a hammer design it is more likely to snag than a hammerless and is more likely than either to have a problem with dirt (again my motto: it may be unlikely, but in a life or death situation unlikely is worse than virtually impossible), unlike the hammered design it is slower and less convenient to use SA so when you can benefit from SA it is harder to use in SA effectively.
My vote is my S&W 65 (.357mag) with 3" barrel (or other K-frame or equivilent Taurus in .357) with exposed hammer AND a Taurus 850 CIA (.38 spl) with the "hammerless" design.
