I'd say the 7950. It ought to handle most anything currently available @ 1080p with ease. The 7970 GHz is significantly faster, but not so much faster that it'll buy you massive amounts of extra time before you feel you need to upgrade -- and when you do feel obligated to upgrade, you'll have an easier time justifying it, because you'll have saved that extra £100 now.
Typically the mid-range is the sweet spot for price/performance, unless you have
really high standards (like, insisting on resolutions greater than 1080p). If you don't have really high standards, then really high-end cards force you pay a high premium for a performance gain that probably won't even apply to you in any practically useful way for a good long while.
Apart from that general point about mid-range cards, though, I haven't said anything that photonboy didn't already put better.
Could answer the question better with a little more information. The cost savings on the 7950 might matter more if you need a new power supply to go along with it, for instance.
@toyftw:
To be fair, the GHz edition of the 7970 is
a little faster than the 7970 listed in your Anandtech comparison.