Yeah I have one advice, get a Gixxer.

Okay seriously, I've never ridden either bikes so your guess is good as mine.
If I had a choice between the two, I vote RC-51. Tiller is a great sportbike but it really can't be compared in the same light as the legendary RC's. I guess, I could be asking myself if I wanted a FZ-1 or a R-1. Tiller is not as aggresive but still retains all the features of sportbike, which makes it an idea bike for most riding trips. RC-51, forget it. I sat on it many times and its the most uncomfortable race-repli of the bunch.
I also heard of Tillers having a problem with FI. When backing off the throttle, the bike behaves like the throttle has been completely cut off,therefore; it wants to drop lean into the corner. I have no idea if its true though, just a hear-say
Good luck