People forget that Moss had some real concerns outside of his character as a prospect. People didn't believe he was strong enough or used his hands well enough to beat the bump and didn't work well across the middle, making him one dimensional.
Of course, it turned out his one dimension was good enough to keep corners dropped way back, and he had such good deep ball instincts that practically no one could contain him for very long.
If we're talking who was the better prospect, it's Calvin Johnson by a large margin. It's all well and good to say you'd take Moss over him, but anyone who actually followed that draft would tell you there's no comparison in terms of prospect rating.
As for right now, despite Moss' ridiculously crappy play towards the end of last year, I think he can still beat almost every NFL corner if motivated. Obviously, he wasn't going to be motivated in Oakland, but he might add a whole new deep ball aspect to that Patriots offense. If he's lost a step, then it's only to move from the most dangerous to one of the most dangerous.
Calvin Johnson is going to have to adjust to the pro game, adjust to Martz's offense, adjust to Jon Kitna, and adjust to Detroit. Right now he's not on Moss' level, but he has all the tools to be there in a few years.