Hello everyone, first post here.
I got into PPPL too! I'm super stoked as it was my top choice(along with UM @ CERN) and was definitely not expecting to get an offer because of my overall GPA. Hopefully I'll see you there Hshc.
From what I've gathered, REUs and SULIs are pretty much equal in value and seem to be a pretty similar experience. Do research for a professor, write a paper, and present your work(usually w/ a poster) at the end. It seems that the better program would depend on what you are able to get out of it, and perhaps the prestige factor, but that varies from program to program anyway. For me, PPPL is the best I could wish for because they are so big in magnetic confinement fusion in the US.
So yeah, you will DEFINITELY work with faculty members. You will be assigned an advisor for your project(whatever they decide to throw you into, ha) and... that's about as far as I know so far!
I'm curious, where else were you hoping to get an offer from?