Generally below 42" go for LCD, 42" and above go plasma.

LCD TV's tend to be brighter than a plasma, so I would consider this if using in a bright room, if the room is dark then a plasma may be better. Plasmas have better viewing angles, so if you have chairs to the side of the tv, plasma is more viewable. Plasmas also show better blacks and whites than LCD.

Plasmas use more energy than LCD, and are a lot heavier too, something to consider if wall mounting. However I've got my 50" plasma on a stud wall with no issues at all. I also went from a 1080p 42" up to a 720p 50" and I didn't notice any loss of picture quality through the HD.

However, If i were currently in the market for a new telly, I'd wait a year for LED to come down in price. They are about an inch thick, light as a feather, use a fraction of the energy than LCD or plasma, and the picture quality of them is stunning

Hope that helps!