Was at Morritt's last July and used OF. My second time with both. here is a run down.
All shops at that end run 40+ft boats. You will be glad to have that size boat if the water gets rough.
Ocean Frontiers will have better service and more experienced divers. They set the standard for customer service. Been diving since 1990, about 3 trips/yr, never found another place that matches their service. Once delivered, never touch your gear again unless you want to. They carry it, rinse it, hang it, lock it, put it back on the boat for your next scheduled dive, setup the BC and gear for you. You just show up and dive unless you want it different.
You have to drive to their office, which takes maybe 5 min. You can dive your computer, head off on your own with a buddy, stay down as long as you have air/time. If you want a tour with a dive guide, they provide it.
Easy to setup Stingray city or anything else you want. I did S.R. city and loved it...again.
If you dive with the shops at the Timeshare resorts, you get timeshare divers. They dive rarely, use air quickly, move quickly, its often family's with kids. Often divers that are new to diving.
No complaints with new people diving and their activities. Just a question of what kind of diver are you and what kind do you want around you when diving.
For me, its no comparison. I would dive Ocean Frontiers but they match what I love about diving.
That time of yr, most dives will be south of the island. Its all based on the wind and can vary every day. The night dive is not so interesting. Done it twice, would not do it again.
You will want at least a full 3mm. I move very slowly, often spend 3-5 minutes in once place for a photo, generate little body heat, use gas slow and stay down long. I would take a 5 mm and full thin hood and gloves. Your experience may be different.
OF can tell you the ave water temp both now and projected. I found my emails to be answered quickly and efficiently.
I likely did a trip report last July or August if you want to search for it.
Pics from that trip are posted here.
http://www.shiningseastudio.com/Underwater/Grand-Cayman/30498948_BWpd7w#!i=2649003262&k=mD2Fkk9
Note: Silversides will NOT be in the carverns during your visit unless a miracle happens. They usually show up in summer.