I found this little factoid on USAToday.com (http://www.usatoday.com/weather/wdensity.htm
). It seems to me that it all depends, but it gives a good explanation of why cold, dense air is better for somethings than others.
"More dense, or "heavier" air will slow down objects moving through it more because the object has to, in effect, shove aside more or heavier molecules. "
"Cool, dense air slows a race car, but some race cars gain from dense air. Cars designed from the wheels up for racing are really like upside down airplane wings that the air pushes down on the track, increasing their grip going around curves. Denser air pushes then down harder. "