I did a day with STT last year at Jersey. What I noticed.... (from the "I" group)
There is a much bigger disparity in range of talent in STT's "I" group. The slower riders were too slow to be safe at times (not just slow, but difficulty holding their lines, getting spooked easily, not being consistent), and the faster riders were way too aggressive. The coaches were there, but they didn't go out of their way to seek you out or be helpful. Passing is allowed inside and outside, but must give a 6-foot cushion between you and other rider (might not be SIX foot but something like that... either way, it was not strictly enforced...). Things got a little hairy when there was a pack of bikes all fuster-clucked together, with the fast guys being too aggressive and the slow guys being too tentative. But, when the track thinned out, it wasn't nearly as dicey.
From what I saw of the "B" group at STT, it seemed like that group was a little more structured than NESBA's. It looked like you had the same instructor/coach for the entire day.
The people in the paddock were friendly enough. The STT folks were too. The biggest difference I saw was the willingness of the CRs to help out, and the structure of the groups. NESBA certainly monitors and controls its intermediate group more closely than STT, which is a big plus, especially when you have the wide a range in talent in that group. With NESBA, I'm solidly planted in the "I" group, but with STT I would be more comfortable in the "A" group.