A .243 is what I use, and you can do more with that than either of the other rounds, and you can do the varmint thing just as well as they do. Also if you can go .243AI or .234WSM and you will have more than either of the other guns. Just a thought I think that you can get the SPS in .243 if you can't there is another Rem700 that is similar that has the .234 in it's selection.

Out of the two options you gave I would do .223 cause you can get ammo for it cheaper just for plinking and it is just more well know and you can get it to do just as well.