I play my 360 on a small 19" HDTV. Sometimes I bring it downstairs and plug it into the 42" Plasma in the living room. For some reason though I much prefer the 19" to play on, and it's even a cheapo Goodmans one compared to a much better Panasonic plasma. No doubt the plasma has a better picture quality, but I just can't get into the game properly oddly.
The smaller screen means I sit pretty close to it on a computer chair compared to 7ft away on a sofa on the big screen. I perform a lot better and enjoy the games much more when I'm close up, yes I could move closer to the big tv, but meh, I like my small one. Just feels more right.
Probably why I don't play games on my ps3 since it's on the big screen for blu-rays mainly.