I prefer pixel in most cases.
For example - default width of content ares: 960px.
In "1024" it will be ok, with small free spaces on the both side of the screen. Put some gradient or background there. In "1280" or "1440" or "1680", or whatever - there will be bigger and bigger gap, but it will look nice.
Yep, for 800 - this site will be ugly. But who cares? Every major players dropped support for 800. There are too few people with this resolution..
There are many various sites, where %% will be much better solution, though.
