Hi there, in 99% of cases, SD content will look better on a lower resolution display (if you view a lot of SD content make this part of your decision factors). The only exception I know to this is the newly released LX range of 50" HD plasmas (1920*1200). These have an amazing processing engine that makes SD material look more than acceptable on this high resolution screen. They are not cheep though, and start at 50 inches in size.
If you need something smaller, pick the best Plasma and best LCD for your budget and audition them both. Take your xbox with you to a specialist, if they don't have one, and try it out.
I am not a gamer, so I cannot comment on either xbox360 or PS3. the review should probably help though. If not, there is another forum on the web where you will get an answer to the smallest detail - avforums. Not as consumer friendly, or pretty as this site, but if its a "nuts and bolts" answer you are after....
At the end of the day, there is a 3rd stage to all this, which is how your eye interprets the image presented to it, and whether you prefer natural or enhanced images. Everyone is different, which its why it is important for you to make the decision. These sites can only go so far, and help you shortlist.
I have my shortlist, but I am just praying that the screen is not to big for the lounge, otherwise, its back to the drawing board!