The plastic tipped aluminum ones seem to be the least prone to heat changes. Of course, with a tuneable, you can adjust for about anything.
I use some cheap Clarkes and Shaws, as they are a) cheap, and b) fairly reliable.
The ones I would NOT recommend for outdoors are not a brand, but generic type.
Brass whistle sound WONDERFUL for studio work. To me, they have a warmer, crisper sound than most other whistles. Unfortunately, they also are INCREDIBLY heat sensitive. Get them outside of a fairly restrictive range of temperatures and they begin to go flat/sharp. I've heard the same of wooden whistles, but have never really played with them.
Anyway, the ones I play seem to sound better in the sun until it hits about 85-90, and then start to suffer from the exposure.
Hope this helps...