Might I ask what GPU you are using? Note that a majority will only allow a third monitor under DisplayPort.
As for the question, you probably won't even notice a difference but that also really depends on the TV's resolution and quality. Plasma is rather strange in one can look amazing via VGA/DVI and then it's twin brother looks like a retarded chimp.
The best bet is to simply try it out and see, I don't think you can really get a solid answer in regards to the question due to the way Plasma can differ on signals.