It goes without saying you should wait a little bit, the 780Ti is out on 7/11, the current prices will probably be cut even further, it's only just the beginning.
Also how can you know which to get before reviews are done on the Ti?
The GTX780s are now going to be $50 cheaper according to anandtech so I'd much rather get that than an R290X, but again, wait a couple of weeks for the Ti