Both of these classes are pretty well balanced at the moment. NIN brings a bit more from a utility standpoint, but both are pretty good at what they do.

As far as difficulty, the rotations are incredibly similar. DRG has more off GCD items to handle and NIN has mudras. Neither of these is particularly difficult unless you have lag issues, which can make mudras pretty hard.

I would choose based on aesthetic. But be prepared for the LOLDRG stuff if you go DRG main. It happens from time to time.