They are pretty different. Also I feel Robben is an actual possibility given the fact Bayern have Goetz and several other good attacking wingers and Ribery prob the most highly regarded of their creative talents.
Ribery is a much more creative player, better passer and also does a great deal of work on the pitch. Robben has more speed and directness which is somethign we really lack. I feel Tevez coming deep and adding to build up and creating is similar to what Ribery does at Bayern but we really lack a guy who is direct and can kill you on the counter with his pace and skill in one on ones (not that Ribery is lacking in one on one skills but they just go about it in a different way and I give the edge to Robben when it comes to just flat out torching his marker). Also I think Guardiola would not give up Ribery but would consider Robben's sale.
Not many wingers in the world would have a bigger immediate impact on our starting 11 then Robben would. His transfer would be a huge step towards us going to the elite level as a CL contender and is a transfer we could probably afford. Most other players of his level would certainly be out of our price range. He is a player I have dreamed of joining Juve since his first season at Chelsea and I think now would be an excellent time to test the waters for him.