We dive with Lahaina Divers (to Lanai) when we go to Maui every year. After having gone to Lanai in a 32' Aluminum V-hull boat once, I will stick with the big, comfortable 46' Newtons that Lahaina Diver runs. The trade off is that they are geared to the less experienced diver, they have AL72 tanks (you can get AL100s though), and they can have up to 24 divers on the boat. Frankly, we've never had an issue with any of these factors except the 24 divers once on an afternoon charter when most of them seemed to be Discover Scuba folks.
One big difference between Lahaina Divers and Extended Horizons is that LD goes to Molokini and Molokai, as well as Lanai and W. Maui sites. They also have a fixed schedule which means you can book your charters based on where they are scheduled to go on any particular day. The ride from Lahaina to Molokini is a long one (over an hour), but if you've never seen Maui from the water, it is an absolutely gorgeous ride and very enjoyable. Since you said you're a new diver, you should scratch Molokai off your list of options. It's a charter for only experienced divers who are comfortable reboarding a boat in rough conditions and doing mid-water safety stops.
Since you're going during a very slow time (Sept/Oct), you might want to check with each operator to find out how many divers it takes before they cancel (I believe it is 6 with Lahaina Divers). With business way down across the islands and uber-low season, it would be horrible for your dives to be cancelled because there aren't enough divers booked.
Frankly though, I don't think you can go wrong booking with either Extended Horizons OR Lahaina Divers. I'd really recommend that if you're going to dive twice, plan on going to Molokini one day and Lanai another day. You could book with Lahaina Divers to Molokini and Extended Horizons for Lanai to get a flavor of both.
