The answer would be, as is so often: it depends on what you want.
The difference would first of all be, that stationary sponsored magic costs 1 fate point less than regular sponsored magic.
Beyond that, some thoughts:
If the prison is very localized, it would indeed only work in the park, but since that would probably be fairly boring, I would look for a way to make it work at least throughout the city. One idea for that could be, that the ley lines have been manipulated to converge on the prison, but they have to come from somewhere. The City you are in is where they start to meet, and at points where two ley lines meet, things start to happen. Maybe magic is bubbling up uncontrollably, or the places attract certain nasties that start messing with them (intentionally or coincidentally). The job of the character with the sponsored magic would be to fix those things, so the prison is kept intact. Seems like enough of an agenda to work with.
As you say, I would keep things like Intellectus rather limited. In the park and maybe he gets a vague knowledge of where and what is going wrong with the net of ley lines, when he is in the park, so he knows what to look for. You could even go so far as to grant him ritual/channeling in the sponsors style magic always, but he can only draw upon the sponsor, when he is on or very close to a ley line. That would mean he is carrying most of the magic with him, but if he needs to dig extra deep, he can only do so at locations close to the sponsors power.
