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.
Both of them are brilliant players but the only difference i can deduce from them is that Robinho is more of an athlete (eg. quicker, stronger and agile) yet Messi is technically more astute and talented in terms of passing, crossing and shooting
Personally i think Robinho will be a legend. messi ive not seen much but have heard glowing reviews and hes only 18. but when you have fabio capello saying that messi is the most extraordinary talent he has ever seen then you have to think wow! coz lets be honest capello knows his stuff and when he says something like this about a player you have to take notice coz he never really comments on players abilities or you dont hear much (well i have not heard him describe players as he has done here) and this guy has managed Roberto Baggio, Totti, Van Basten a long list of legends so he must see something in messi. i agree with some of the comments made earlier that robinho is already a full international and a first team regular but the future is bright for both and the more of these players that come through the brighter the future for football and us as we will see better quality players.