Reloder 15 is becoming the go to powder for many people right now. I think it's due in part because the military uses it in their M118LR 7.62 round, so everyong has to do what the military does.
Don't get me wrong. Reloder 15 is a great powder. I use it in my service rifle highpower ammo. It is not advertised, as far as I know, to be insensitive to temperature changes, but pretty much all the RL15 users report that it is.
There was a exodus from Varget to other powders about a year ago when a few people noticed a large discrepancy between the varying lots of Varget. I have not personally experienced it myself, and I tend to buy Varget in 1lb jugs.
I still think Varget is a great powder, and so do many others. I use Varget in .223/5.56, .308, 6.5x55, and even .30-30 (in Contender pistol).
If Varget works for you now, there isn't really a reason you should switch. If RL15 is easier for you to get (and cheaper), then you may want to try it out.