The age old question....
It really depends on how deep your pockets are. Put it this way, at the current price the GTX 780 becomes a better short-term investment whereas the Ti is more a long-term unless you have the money to spend.
Then there's the possibility of overclocking, because indeed it's relatively easy to get a 780 running faster than a stock Ti.
Then you have to consider whether you want a reference or custom cooler card. Sure, at reference there's around £100 difference in it, but once you step up to the non-reference cards the prices start exacerbating in difference.
Therefore, as a personal opinion I'd buy the 780 over the Ti.
It's still a fooking fast card!