Don't overestimate the Brown/Risd connection. Students can't always fit classes into their schedules, the two schools are on different schedules, and you can't do Brown classes freshman year. That said, it is still possible to take classes at Brown, as long as you are proactive about it. RISD also has liberal arts "concentrations" in english, social sciences and one other area that escapes me. (Check out their website for details.)
If you want a true liberal arts experience, Syracuse would give you more of an opportunity to explore than either RISD or Pratt.
If you have to go into debt to go to RISD or Syr, it's probably a good argument for going to Pratt.