There was some duplication, but I found both useful there was more general discussion and chat in the NCT classes, but the hospital classes were more focused on kind of talking you through what would happen with a birth at that hospital making sure you knew the numbers to call, when to come in, where to park, how to get a free 24-hour parking pass (ha! if only 24 hours covered it...), what the delivery rooms were like, passing around things like forceps and epidural catheters and other bits of medical equipment so you knew what they looked like, talking through the "what if"s that people had.
For example, it was the hospital classes that said "if you have a ventouse or forceps, the room will fill with several extra people, but don't panic and assume there's a problem -- it's just routine and precautionary" which was a useful tip to have. Also the hospital classes were led by a midwife who was able to give some medical advice (and actually did a few imprompteu checkups on women who were between regular midwife appointments but had some concerns.
And, of course, there was the fact that while I'd expected to get to know more people through the NCT classes than through the hospital classes (I was only really doing the hospital ones because they were free, so I thought "why not?") it was actually the other way round. If I hadn't gone to the hospital classes I'd have missed out on the friendships I made through them -- and it was actually one of the other mothers-to-be there who tipped me off about what is now DS's nursery, which hadn't long opened at the time and I didn't know about, so potentially he'd have missed out on going there (where he is blissfully happy) and on the friends he's made there, and the whole course of his life would have been different... .
No pressure, or anything...