We dive with Lahaina Divers when we're on Maui. Although they will take a max of 24 divers, the boats themselves are big (46' Newtons) and well laid out, so there's always somewhere to go that's uncrowded. Plus for someone with motion sick issues (I wear a patch when I fly or dive), the stability of the big boats is a huge comfort on the 30ish minute crossing to Lanai.
We've never felt crowded and they break the dive groups up into 6 diver groups. We've experienced full charters on just about every afternoon "turtle reef" charter we've done since that's when they take the Discover Scuba folks, so we don't do afternoon charters anymore. We've also had as few as 12 divers on more than one charter and those charters were wonderful!
On morning charters they serve sandwiches between dives and on the way back to the harbor, they serve warm, fresh cookies. Yum!
We've always found the DM's and instructors to be great guys, very safety minded, and our certification classes with them were first rate in the skills we were taught and the high standards they held us to.
I think the big reason we've not dove with Extended Horizons has more to do with the fact that they both go to the same locations, run the same dive profiles, but Extended Horizons is $20 more per charter than Lahaina Divers. Plus, now that we get an automatic 10% off since we got certified by Lahaina Divers, that makes the cost difference even bigger for us.
One complaint I've heard is that LD uses AL72 tanks vs EH's AL80s. Frankly, with the profiles that both ops run, I've never gotten close to sucking an AL72 dry on a charter. OTOH, on many of our charters our DM has had to take divers back to the boat early while we continued our dive. One guy was so bad that for the 2nd dive the DM's switched him to an AL100 so he could stay under for longer than 30 minutes!!
We'll be in Maui the first week of April and have 2 Ed Robinson's 3-tank dives booked and 2 Lanai charters with Lahaina Divers booked.
Frankly, I don't see how you can go wrong with either op. I've always heard good things about Extended Horizons and have never personally experienced anything bad with Lahaina Divers, so pick one and go dive!!