Difference between the 17HMR ad 17M2 is power. The M2 is a necked down 22LR, and hard to find. I've only seen them once for sale in Ontario. The HMR is a magnum, easy to find, but expensive(for a rimfire). I've only shot an M2 once, and wasn't impressed with it. The HMR is a fun little round, and great for tearing up gophers.
For plinking, the 22LR. It won't burn your wallet as much as the magnums will. But, if you want to go to the range with a magnum, pick the one you like best. Surely someone there will let you try out their magnum rimfires. As for woodchucks, the 17HMR. That little red poof is mildly entertaining.
You already have a semi auto, try a bolt action. Again, what calibre do you want? How much you willing to spend?
When hunting for meat, I try to get head shots as much as possible, and fortunately have improved my skills over the years. What I use is Federal bulk or Winchester Dynapoints. When I go to the range for general plinking, this is what I bring. Cheap, and available. I know how they shoot out of my guns, and I know what to expect from them. I also hunt primarily with the 22LR or 22WMR. 17HMR is more for varmint carnage.
to rimfire! 
