If you can sell a 780Ti for $300-350, you can get dual R9 290 XFX for
$440. (2 rebates per household). If 300 series and GM200 force major price drops, or bring massive gains, you can always resell each 290 for $100 each in 12-15 months, losing barely $200 in resale value making your cost of ownership very low.
Some of those XFX 290 cards can also unlock to a 290X! Ask
WhoBeDaPlaya how to perform the unlock.
Even MSI Gaming 290X is just
$260.
The only reservation I have is if you heavily play many GW games. In that case, maybe it's better to get dual 970s for you. 3.5GB is still more than 3GB on your 780Ti.
Still, selling a 780Ti for > $300 and getting dual 290s for $440 sounds like a very cheap stop-gap solution until you upgrade again in 2016.