For concealed carry, I definitely prefer hammerless, or else a shrouded hammer like the 638 has. I think cocking the hammer is for target practice only. In a life-threatening situation you don't want that hammer snagging your clothing, and you don't want to manually cock it because you could very easily shoot someone unintentionally.
For non-concealed uses, I like having the exposed hammer for non-defensive purposes (targets and hunting). I'm in the market for a 4" revolver... still waiting for just the right thing to come along.
As for the internal lock, personally I don't have very strong feelings about it, but I'd rather not have one if given the choice. There have been very few cases of a lock malfunctioning and preventing a gun from firing. But simpler is better and less things to go wrong, plus it will have better resale value (although I've never sold a gun).