Which is a better deal?
Both are great. The 7950 has around 200 more Stream Processors so essentially it is the better GPU. But the XT is usually 70 US dollars cheaper so it's a better deal. They perform nearly identically aswell.
The 7950 is the better GPU, the reason you may get a 7870 XT is if it is much cheaper. But in the UK atm the cheapest 7870XT is around £200 and the cheapest 7950s are in the range of £220-£240 if you look for them.