There are moves you can make in NL that you can't make in limit like pricing your opponent out of the hand by making a bet or raise thet ruins his odds. For example, in limit it's often to call a river bet based on pot odds, in NL you can bet so large that the player can't call based on odds. I'm sure there are plays in limit that don't apply to NL, I don't play enough limit to know. I wouldn't mind if some of you guys care to tell me about them since you guys play more limit than I do.
I think the answer to the question, which is harder, can also be dependent on the person. Not everyone can play both, some people play one better than the other because it is a better fit for them.