I've used both in 308. I can get 20-25 fps more speed with RL-15. Accuracy is about the same, which means very good. Temperature sensitivity may never be an issue, but I prefer Varget because of it.
It can be 110 here in August when shooting at the range, I've hunted in single digits in December. That can be 150-200 fps difference with some powders. From my reading Varget would probably only see 50 fps or so difference with the same temperature swings. I'm actually more concerned with a safe load developed in the winter being over pressure in August. Temperature sensitivity works both ways.
