I wouldn't worry about the heat of 1 plasma unless heat is already in issue in your room. Newer plasmas may be better in that reguard as well.
By the word of cnet the 8500 series is the best LCD they've tested. Their main gripe looked to be poor off-angle viewing. Personally my LCD is the same way and it's a bit irritating as I sit too close. The center looks good, but the sides have a fairly noticeable color shift.
Cnet also gave the G10/V10 plasmas some pretty high marks and are much cheaper than the grossly overpriced 8500 series.
Though if you're looking to buy in a few months, might be best to wait for the 2010 models of LEDs and plasmas to be released. Local dimming LEDs need to come down in price and Panasonic needs to reach the black level of Pioneer. Be nice if both happened.