I've had about equal results with TAC, 4064, 4895, RE-15 and Varget. 3031 ain't a bad powder either As mentioned above you may have to use what you can find and I'm betting you can work up a good load with any of them. The 308 is the least picky round I load for. Almost any powder and bullet gives good results.
I like Varget a little better simply because it is less effected by temperature extremes. To be honest that is not a huge concern, but if it shoots just as well as the others why not use it. I may find myself having to shoot at extreme temps someday.
From about 30 to 80 degrees, temperature simply isn't a concern. If you are shooting near or above 100 you can find an otherwise safe load is over pressure. Once you get to zero and below velocity could be 150-200 fps slower than when at 80 with other powders. There will be velocity variances even with Varget, just not to that extreme.