I am not positive but I think if you have the tripod basestation you can have RTK anywhere. I know a couple guys in our area that have their base station on their grain legs and are able to cover most if not all of there farms especially with the newer radios. But the portable base station should work anywhere. As for the Trimble vs JD I am not sure. I know trimble has two different levels of accuracy under OMNI Star. Don't hold me to it but I think the XP is 3-4" pass to pass and the HP is 2-3" pass to pass. I'm guessing the more accurate one would have a higher subscription fee and less coverage area but I'm not sure. I know that JD SF2 is + or - 4" pass to pass. As far as subscription fees and unlock prices I really have no idea.
Another option might be Trimble's new service CenterPoint RTX. This operates just like the omnistar, not requiring any base station, radios or cellular corrections like rtk, and offers repeatable 1.5" accuracy. I think it is only compatable with trimbles more expensive displays, the CFX-750 or FMx. It has a more limited coverage area when compared to OMNI Star but it does cover most of the midwest. Again I have no idea of the subscription and unlock fees.