I actually bought Ray Rice in a couple of leagues this year based on that perception. I did get Pierce as part of the deal, but still feel we have several good/great years left from him.
My vote goes to Ray Rice. It was scary owning him in so many Dynasty leagues last year and watching Pierce come in and be just as effective if not better.
Rice is significantly more dynamic than Pierce. Pierce, to me, is just a pretty good RB, I would even call him above average. Nothing wrong with that. I think he can be very effective as a wear them down kind of guy, he runs strong. I don't see any great agility and he is lightly used in the passing game. I think Pierce seems a bit overrated on this message board.
Rice got 254 carries and 61 receptions through 15 weeks. That is 21 touches per game, and he actually got a bit more per game from weeks 9-16 than he did in weeks 1-7 (not including week 17 when Baltimore sat their starters). I have no problem with that level of use, and I think he will be right around there for several years.
Actually, Pierce appears more dynamic to me. He isn't going to take the job from Rice due to his versatility and reliability, but Rice isn't a dynamic player. The other point to remember is the jump that 2nd year players take when the game slows down...Pierce is one to watch.
The problem is you will just have to be patient. As for selling high on Rice, I think it may be worth checking into, but certainly hard to trade a player that productive.