If you want to be a one-gun man, I would stay with the 300 WM. and yes, use it on prairie dogs. When I was a one-gun man, I used to walk around and thru PD towns with my 35 Whelen AI. It was a great way to learn range estimation and position shooting. It will give you a lot of familiarization with the gun and your capabilities with it. If you are not totally happy with your current rifle, put your money into upgrading it. True up the receiver, match bbl, new stock, or new glass.
I've had my Whelen since 1976, and have shot everything from PDs to elk and caribou with it. My longest shot was over 425 yds, taking the heart out of an antelope. Three people were telling me it was too far, and not to take the shot. You will have one gun, and know your limitations with it, and also know what you can do with it.