IMHO, you can't even compare the two.
5070 is last year model. 5080 is this year model under the code name Kuro - it has new plasma cell structure or something like that.
5080 is so much better that not only it outperforms 5070 it also outperforms every other plasma in its price range including some 1080p Samsungs and Panasonics.
IMHO, Costco sells 5070 for very high price. Walmart sells 6070 (60"!!!) for just couple of hundred more.
You can buy 5080 for the same price for sure almost everywhere and with some luck and FS/BB sales even couple of hundreds less.
Check HDTV Deals forum.