DH and I just got back and we did 2 excursions (3 if you include the Paradise Cove Luau, which wasn't adventures by Disney). Attendance wasn't an issue (that I know if) with either (a lot of the people in our groups signed up there, and our guide was genuinely surprised that I had booked the excursions ahead of time by the phone).
We did Hawaii Fire Surf School and I cannot speak more highly of it! The Firemen were great (and so was our guide). I've compared my surf experience to some other's that learned to surf through more casual instructors and they've all been quite jealous of our amazing time with Hawaii Fire. If you want to surf, then I can't recommend them enough! I'm assuming the price difference between the prices is that with Aulani's excursion you get transportation and snacks (which was really great, since we were exhausted afterwards and I would have hated to drive back!).
We also did the Kualoa Ranch excursion which was okay (although, the bus ride there and back was amazing because of our driver, who kept us entertained with stories of Hawaii the whole time). I think my lack of enthusiasm stemmed from not having much knowledge going into it. It was a lot of time at the "secret beach" to hang out, swim, snorkel, etc. The Catamaran was a decent size little boat (no little mesh area to sit) and it seemed like the boat ride was really quick. Then the horseback ride was two hours long! It was a good time though.
I'm working on a trip report, but still trying to fend of the jet lag! A big shout out to our guide, Eki though - he was lots of fun and full of information!