They're essentially the same GPU with the Titan Black having double the available memory. Nvidia charges out the ass for memory, so my suggestion would be to wait for the 8xx series and see if Nvidia has any reasonably priced 4GB (or more) GPU's that perform on par with a GTX 780/R9 290 or better. 3GB of memory is going to hurt GTX 780/780 Ti owners sooner than later once proper next gen games come out and the minimum gpu requirement climbs drastically from 2GB to 4GB for 1920x1080. You've waited it out this long, a few more months won't kill you.
what he said.
We should have some proper benches/pricing on the 8xx in the next few weeks