The cooler air is better for combustion and performance. Colder air/hotter air really doesn't impact mpg once the bike is warm.
In colder weather you will have decreased mpg due to the ECM "choking" the engine and providing a richer mixture until the bike is warmed up. Once idle has returned to normal then you should, within reason, getting about the same mpg.
However, if making a bunch of stops you will be restarting a "cold" engine many times.
All that said, my experience has been that long trips in cool weather provide similar mpg. Short hops in cool weather provide lower mpg. Any trips in extreme cold will significantly reduce mileage because the bike really doesn't warm up. Then again, I don't ride in those temps, I typically stop riding around the freeze point.
Hot weather. Extreme hot you may see some reduction in mpg but that's mostly due to perfance reductions at those hot temperatures and just more twisty of the throttle.
2009 1700 Nomad
Candy Diamond Red/Pearl Luster Beige