Both are "signature" Maui Dives. Extended Horizons only dives Lana'i - the Cathedrals and other sites. It's a wet ride back as their boat is smaller. The other option in Lahaina with their own boat(s) is Lahaina Divers. They have 2 big Newton dive boats so will have more divers on-board typically. One DM per 8 divers is the law. One advantage they have is they do both dives on alternate days - they have the range to run to Molokini also. I got lucky and caught a trip with them with 8 divers. Not likely in August though.
Molokini is a 1/2 sunken volcanic crater. Inside is really fishy diving - the snorkel catamarans go there and tourists feed them. The better (and deeper) dives are off either point. The Backwall is an advanced dive, often a live drop with some current. It's pretty vertical and the hard bottom is about 350-400' down. Better chance to see bigger stuff there although we've almost always seen white tips inside the crater resting in the sand heading back towards Maui.
You might want to book boat dives before you get there also.
Other options for Kihei dive operators are B&B Scuba and Mike Severns. I like B&B and Ed Robinsons equally, never dove with Severns but they have naturalist DM's so you learn stuff on the ride. The B&B's - Brad and Blesi - are nice people. They check-in early though - 5:45.
Where are you staying? It makes a difference since the drive between Kihei and Kaanapali is about 45mins. Notice the departure times (5:30-6:30AM) for the Kihei boats - they do that to beat the tradewinds coming back which pick up late morning. One road and even early morning it's busy with resort employees getting to work. Lahaina Divers checks in at 7am, the Advanced Lanai dives at 9am. They also have an afternoon dive. Robinsons does a 3 tank Advanced Dive if you've got the cert. or experience.
There's good shore dives in both areas also. In Kihei there's Maui Dream Dive Co. or Shaka Divers. Shaka Doug is on the board here. Both do escorted paid shore dives. B&B, MDDC or any of the Scuba designated Maui Dive Shops (not all of them) rent tanks/weights. MDDC at one time had a buddy sign up sheet also. If interested maybe call and get added to it b4 you get there.
In Lahaina, Lahaina Divers also does shore diving. They also do the "collapsed pier" (Mala Pier) mentioned above at night from their boat. The legal entry there is difficult around the coral so it's an easier way to dive it. Another option for guided shore dives in that area is Tiny Bubbles Scuba.
Here's a list of most of the shore dives:
Scuba Shore Diving Site Listing for: Maui, Hawaiian Islands
In August I'm in a t-shirt and boardshorts. But that's me even when others are in a 3Mil, I don't get cold. And I do a lot of dives per day.