What do most who shoot varmints or targets @ 200-300 yards like for magnification on their scopes and why? I have a Nikon 3x10x50 and am thinking of going higher like 3x15.

I've got a 6-18 VX-II.....works for me.  I have found that I like to keep it cranked down to 8, 10, or 12 b/c of mirage and I shoot better at the lower magnifications at that range.  This past weekend I was shooting at ~400 yards with 75 and 77g BH and I was getting <=3" groups on 10x.  At 18x they opened up to around ~4".  I attribute this to any slight movement being more noticeable at the higher mag.

Rifle is a 24" RRA Varminter.  I was shooting prone with a bipod...surrounded by cattle claymores.

ETA: I also did some shooting at ~950 yards and the 18x helped alot.....for those occassions it is nice to have a higher mag.  I think the 6-18 is about "perfect" for my purposes with that rifle.

Good Info. I noticed I have now turned it down from 10 to about 8 for 200 because 10 wasn't enough and the movement. Looks like the groups have shrunk by 1/2" or more.  I just wonder if I would go up to 15 if I could at 200. Sounds like not.

On my varmint rigs I use 6x24 variables. These are for shooting at 200 yards for the shortest shots, and targets are roughly the size of a soda can.

Don't cheap-out on the scope. the cheaper ones will not have the clarity you'd like.

I never cheap out, thats not my style. Thats why I have a Nikon.  I reasearch and buy what I feel is the best bang for the buck, even if it is a lot of dollars. I don't want to say what I spent on my set up.

I have used various 3-10's, 4-14's, and 6-24s--all on prairie dogs.  I have never cranked-up any scope beyond 16x for more than 1 shot due to mirage and the difficulty in keeping on target during recoil at that high a magnification.  Most shots are at 12x or so and that is out between 200-500 yards.  I won't ever spend the money on "high magnification" again.  Use that money and get quality glass with snap and contrast and 12-14x will be all you ever need for prairie dogs with a .223