RE: .204 or .25 WSSM?
Why dont you think about the .243, it's really the king of varmint rounds. Its heavier bullets stay on path and is very flat shooting. you can also use the 55 gr ballistic tips that are pushing right at 4000 fps and is the flattest shooting round available.
Heres a comparo.
velocity
----------muzzle----100yd------200------300-----400----500
.243
55gr------3910-------3493-----3114----2766----2444----2144
.204
40gr------3900-------3482-----3103----2755----2433----2133
energy.
.243
55gr------1867--------1489-----1184----934----729-----562
.204
40gr.------1351-------1077------855-----674----526----404
trajectory
.243
55gr.-----------------+0.7--------0.0------4.3----13.1----27.7
.204
40gr.------------------+0.7-------0.0------4.3----13.2----28.1
I didnt post the .204's 32gr. cause it ballistics arent as good as the .40 gr.