I have seen Messi play about 5 games and i have seen Robinho play a few more than that. It's hard to say at this point because much of Robinho's play was against the Brazilian league teams, and it can be hard to judge the quality of the non-powerhouse teams there. Further, i have seen messi in one game for Barca and a few for the U20's of Argentina.
From what i have seen, Robinho is better when it comes to one on one ball skills and breaking past a defender in order to create space. He has also demonstrated a solid sense of vision in being able to find his teammates and know where they are.
Messi, on the other hand, IMO, is better at taking the ball in traffic and suddenly charaging up the field and carving/slashing through defenses as a whole. There were some moments in games where i was left thinking, "How the hell did he get from point A to point B so effortlessly?"
I think we'll see more Messi this year but not as much as Robinho. All i know is this rivalry of RM-Barca just got alot more interesting with two of the best young talents in the world.