IMO it will be in your best interest to get up early and take the 45 minute drive down to the Kihei boat ramp to catch a boat to Molokini and S. Maui sites. The most often recommended here are Ed Robinsons, Mike Severns, and B&B.
I think if you want pretty much a guarantee that beginners won't be on the boat, you'll want to consider booking either Ed Robinson's 3-tank charters on Tuesdays and Fridays, and/or their Adventure charters which are Sundays and Wednesdays. Both charters are marketed to the more experienced and independent diver and the dive sites/profiles typically reflect that.
A big caveat would be that if Ed Robinson's doesn't have enough divers to send 2 boats out on Sundays or Wednesdays, they will often combine their regular charter and Adventure one, so it's not a bad idea to check a day or two before if that would bother you.
Regardless of whom you choose, you'll find that most operators do a good job of trying to separate their divers into like groups, so although you might have a new diver on the boat, you won't be likely to have them in your dive group unless you're the only experienced diver on the boat!
