The 700u is one of the highest rated plasmas available, so you really can't go wrong with that model. It has also been through its stress test among community members and various hardware sites. The 80U is still unproven, so you'll be entering uncharted territory with that television. Either way Panasonic is a Tier 1 company, so you really cannot go wrong.
I would go with the 700U simply because it is 1080P, has a better sound system, and is a veteran that has earned the respect it is given. Panasonic's website allows you to easily compare the specs of both TVs.