I recommend the Westinghouse for sure, as not much can output WQXGA, and on top of that the Dell only has DVI and VGA rather than the Westinghouse that has HDMI, 2x DVI, VGA, 2x component, (and a few 480i connections i didn't count 'cause I don't use them). I may be a bit biased though, since I own that exact monitor. It works awesome for the 360, and looks really good at 1280x1024 16:9 mode. It'll be even better once the fall update rolls around. Good luck finding one, they're selling like hotcakes.
oops, edit:
It works really well for the PC too, there's just one thing it gets tripped up on: If you have an NVIDIA card, it gets tripped up on the latest series of drivers and only displays 1080i through DVI instead of 1080p. Just get slightly older driver versions and it'll work fine.
You'll have to sit 3 feet away from it though, it makes for one HUGE computer monitor!