I am thinking of buying a gtx 980 but the gtx 780ti is £100 cheaper so is the gtx 980 worth it?
As far as I know 980 slightly outperforms the 780ti but uses less power, Logan has mentioned that he's waiting for the 980ti to come out before upgrading his 780ti card(s).
Short answer if you need an upgrade now go 780ti for the savings and put that towords the 980ti, use it for a few months, when the 980ti comes out sell the 780ti and recoup your costs + savings\
Just my 0.02 cents
You will run into Vram problems with the 780ti.
Not having a NVidia card since the 460, I'm probably not the best reference, although my 7970 managed to play Batman AA, Skyrim and the new Wolfenstein @ 5760x1080 at 60fps (Stock Settings) no problems, 780ti is a newer card, but those with 780's may beg to differ
Also what Res will you run at, 1, 2 or 3 Monitors. Will you be strictly Gaming or video editing, what's your CPU?