I didn't interview for both (just New Pathways), but I picked up a bit about the HST life...
NP seems structured to give you a good amount of free time early on (2-3 afternoons a week, I'm forgetting which) with the idea that you'll use most of that time to do other things that will benefit you as a person and (eventual) doctor. Clinics, volunteering, shadowing, research, a major hobby/interest, etc. A lot like what UPenn does as well with their schedule.
HST kids seem to have more consistent work and a more consistent schedule. I'm not sure about how the curriculum differs, but one first-year I met was trying to switch _into_ HST. He didn't really like the small-group thing, so I guess HST doesn't teach in that way, at least to the same degree. One of his friends said that HST was a better fit for people who want and need more structure and feedback. NP is pass/fail for the first two years, but maybe HST isn't.
Ok, as you can tell I don't know many facts about HST, but these general impressions might make more sense when you learn what's up. I will say, though, that the HST program had rented out a theater for the third Matrix for their kids and guaranteed them partially-subsidized tickets. Apparently, the HST program has bucketloads of money. Other people have said that it seems the HST kids are treated better. My guess is this is all about the funding - the NP kids are divided into those societies/houses, while HST is its own society.
Good luck at your interviews!