They both use non-reference designs with better power systems so they can be overclocked and over-volted. Note that the cards themselves will run cooler because of superior fans, but they may heat up the case temp a bit. If you have good case airflow, this shouldn't be a problem.
for sc2 and wow def the gtx 570, I got mine for 304 AR, but if you cannot afford that then the gtx 560ti
def do not get 2 6850s the avg fps will be high but minimums are crap, not a good idea and I'm speaking from personal experience with the 6870s in crossfire..
overall I'd get a gtx 570 if you can find a good deal on one
my second pick would be to get a 6950 but personally I'd rather pay the extra money to get a 6970 to ensure that level of performance not to mention that some flashed 6950s are having problems some time down the road so personally I'd rather just go straight to the 6970 if your going that route or you can gamble with the 6970, but generally blizzard games run better on Nvidia gpus