In my opinion the AR-10 will cost you more but will allow for some pretty quick follow ups. Why not build the 6.5 on a bolt gun and have the best of both worlds? I think the 6.5 is the cartridge of my choice in this situation.
you have a varmints worst nightmare for sure. A friend of mine created a similar wildcat years ago and it is perfect for groundhogs even if the wind is blowing a little it screams right to the target. congrats and I dont blame you if you forget about the BR!
I love my swfa 5-20hd, so any option that gets you into one of those sounds good to me! I have been looking at the 3-15 as an option for my next build.