Can signal quality be affected by high temp?
Propagation of UHF and VHF signals can definitely be affected by weather.

In particular Tropospheric propagation (radio waves traveling through the Troposphere, the lowest portion of the earth's atmosphere) and Tropospheric Ducting are seasonal:


  Tropospheric ducting is a type of radio propagation that tends to
  happen during periods of stable, anticyclonic weather. In this
  propagation method, when the signal encounters a rise in temperature
  in the atmosphere instead of the normal decrease (known as a
  temperature inversion), the higher refractive index of the atmosphere
  there will cause the signal to be bent. Tropospheric ducting affects
  all frequencies, and signals enhanced this way tend to travel up to
  800 miles (1,300 km) (though some people have received "tropo" beyond
  1,000 miles / 1,600 km), while with tropospheric-bending, stable
  signals with good signal strength from 500+ miles (800+ km) away are
  not common when the refractive index of the atmosphere is fairly high.
  
  Tropospheric ducting of UHF television signals is relatively common
  during the summer and autumn months, and is the result of change in
  the refractive index of the atmosphere at the boundary between air
  masses of different temperatures and humidities. Using an analogy, it
  can be said that the denser air at ground level slows the wave front a
  little more than does the rare upper air, imparting a downward curve
  to the wave travel.


As to which is "better", UHF or VHF that depends on your goals. Lower frequencies generally travel farther while higher frequencies are more affected by rain and snow. Often you do not want your signals to travel too far as in the case of a Wi-Fi router or cordless phone in a home for example.