Just up to you. Black levels on plasma have generally been better than those on LCD, but LCD has been doing better lately. On the other hand LCDs generally have a less reflective screen than plasma. If you're going to be sitting very close to the set, then 1080P might be better, but at the distance most folks watch from, with a 50 inch or smaller screen, you're not likely to see much difference in resolution. I have a 50" 720P (768P actually) Panasonic plasma, and the picture looks great.
They're close enough in size that the viewing area isn't really a factor. It's hard to judge in stores, but I think you should go with what looks best to you.
You can't trust in store set ups. I've seen tvs in stores that are great tvs that look like crap next to not so great tvs. You will have to calibrate any set once you get it home for optimum performance. Go online and check the specs and reviews of each set. Go to websites that do independant reviewing and not the Vizio site. That should give you a better grasp of the differences between each set and you can make a better decision. Good luck.