I'll toss in a couple of comments from my experience. I dive 3 people (1 tank) or 2 (2 tanks) from a 10 ft PVC airfloor inflatable with a 2-stroke 15HP motor. That motor will plane the boat, but that weight's about the limit of what the boat can handle and we dive within 1/4 to 3 miles of launch, within swimming distance of shore. Advantages: boat and motor each weigh <<100 lbs and boat folds into a smallish roll for easy transport, no trailer needed.
I also have a couple of 14 ft inflatables (one PVC, one Hypalon), with aluminum sectional floors. You can easily float 4 guys with gear for one dive. The hard(er) floor makes these better boats, but don't kid yourself, you're not going to like running 10 miles to the reef in any flat bottom boat, much of the time, especially if at all loaded down. Haven't owned a RIB, but they're bound to be better riding boats, and a lot of divers like them. My advice would be to keep it simple with tiller steering - no helm to waste interior space - but that's a personal preference. Oh yeah, I have a 40 HP fuel-injected Nissan 2-stroke that's plenty of motor for these boats, but I wouldn't go smaller. Nissan/Tohatsu motors are top quality but not common in some areas.
With inflatables Sun/Heat = Death, so keep the boat under a stand-off cover whenever possible. This will bring flames, but in my limited experience, PVC will last longer than Hypalon if you don't have to store it out in the direct sun. There's a reason you get longer hull warranties on PVC boats than on Hypalon ones (this used to be true, haven't checked in a few years). The Hypalon fabric is more sun-tolerant, but the really key factor is that the glue isn't, and every seam in a Hypalon boat tube is glued. In a PVC boat, only the floor and transom are glued in, the hull seams are welded. My PVC Zodiac is 25 years old and there's not a curled edge or leak in any welded seam anywhere, truly permanent seams. The glued transom and floor however, have fallen out. My Hypalon Achilles is only 19 or so but lived many years out in the sun. Many seams are going, one night anchored in calm seas with the motor up pulled the transom loose, and the Hypalon fabric is delaminating everywhere. Sun and time are not kind to either fabric. Since I don't live in the tropics, I'll be buying another PVC boat this winter. Hypalon is reportedly more abrasion resistant, but PVC is plenty strong and tough. We have rocks here, not coral, and my PVC boats hold up fine.
Last bit - unless you're going to run the boat lightly loaded, get the dealer to swap the stock prop on your new motor for one with smaller pitch. 40 or 50HP doesn't make a rocket sled if there's any weight, and motors are usually sold over-propped for real world use. You won't miss the top-end on the ocean. Get a portable depth finder, handheld GPS, EPIRB/Personal Locator Beacon, first aid kit, dry box/bag, anchor, and drift line and you're ready to go. (Not really, there's more to it, so do some reading and learn from experienced boaters)