Hello,
One major thing you will want to take note of is that June-November is hurricane season in the Caribbean. The chances of one hitting while you are there is not great, but if it happens your diving may be ruined, or worse. For that reason I would personally avoid Cancun/Cozumel and Roatan/Utila during this time.
Should you choose to go to Roatan I highly recommend CoCo View Resort (http://www.cocoviewresort.com/). I've been there five times and would never consider going with another operator there because CoCo View is 100% dedicated to diving (there's not much else to do on the property) and as such doesn't get many "tourist" divers and does not allow children.
There is no diving to speak of in Cancun. Across the channel is Cozumel, but it is all drift dives, which may or may not be your thing. It's been 20 years since I've been there but I remember it being crowded and I have no desire to go back.
Baja is a great option for this time of year, and flights from Vegas should be easy. The Sea of Cortez should be warm by the end of July/beginning of August, and this also happens to be sea lion pupping time. I would discourage any serious divers from going to Cabo and instead direct you to either La Paz or Loreto. In La Paz I've done a few trips with Baja Expeditions (http://bajaex.com/) and they are a great outfit; other Wetpixel folks have recommended them as well. The Los Islotes site you've mentioned is off La Paz and is a popular destination for local dive boats. The caveat here is that almost all the dive sites are a 1-2 hour boat ride from the town.
Loreto is a bit further north up the peninsula, but is situated in a marine park. Dive sites here are typically within a half-hour of the harbor. I've been diving twice with Dolphin Dive Center (http://www.dolphindivebaja.com/) and they're another outfit I'd highly recommend.
I haven't yet dove San Diego but it will give you a very different type of diving than any of the other sites. The water is cold, for starters, but that doesn't make it bad - just different.
Have fun.
-Gina