I recently got a 5500 and love it. It's plenty fast for me (I use it for games, office, and classroom stuff). I carry mine to class most of the week. If I turn the GPU down, I can get 1.5-2 hrs on the battery. As far as screens go, I get comments on the clarity of the 5500 screen all the time.
The size is different-the 7700 is 2inches wider, a half an inch thicker, and has a starting weight (w/o battery) of 2.5 lbs more. The 7700 is a beast. Also, the 7700 is quite a bit more expensive.
I think the 7700 is a badass comp., but I couldn't justify spending that much money. For what I need, the 5500 delivers on flawlessly.
You'll enjoy either one---just depends on how high performance you need it.