The 223 Remington would easily be flat enough for those distances, and the ammo would definitely be cheaper.

For the price of a Pro Hunter barrel I would look at an MGM custom barrel. You can get the length, contour, and chambering you want. Also if you are really concerned about flat shooting you could go with a 1in14 twist and shoot the 40 grain bullets in your 223, and that bullet can approach 4,000 fps. Especially if you can get your hands on some good reloads. But you can get some Black Hills 40 grain ammo which should do about 3800 fps(just a guess).

Personally I do not care for the killing ability of the 204 Ruger. We have tried a couple of them on WV Ground Hog's and we had too many crawl offs for my liking. But this is just my opinion on the 204, I have talked to a few guys who really like them. Tom.