The bigger 50 mm will let more light in. On a varmint rifle your mainly hunting in broad daylight so thats not much of a factor, unless your state allows night hunting for coyotes and stuff.
I own the Leupold VX-III 8.5-25x50 LR with fine plex. I am really fond of this scope for what I use it for, varminting and long range target practice. I haven't had tracking problems that I hear so much about. Its plenty clear at 1000 yards enough to resolve my target. I find no hinderance having a minimum power of 8.5 and while deer hunting a couple of years back I remember how well I could see after the sun had set and was starting to get dark.
The long range series also has more elevation than the Bushnell 6-24. But I see that Leupold just raised their prices in Cabela's so I can see why Bushnell is looking so tempting. I think your on the right track, good luck.