Keep your options open.
If the weather is bad, DiveTech will be going around to the Sunset House side of the Island, so a longer boat ride after you drive there.
You don't need to pick just one unless you are diving just one day.
I have been to both and liked both. I was disappointed at the shore diving at Cobalt Coast/DiveTech as it was closed down due to weather, whereas Sunset House was not affected. I liked that DiveTech has 100cf tanks.
First, when the north shore is calm, you'll be diving the north wall with Divetech. That can't be beat! Sites like Ghost Mountain, Hepp's Wall and Hepp's Pipeline, Princess Penny's Pinnacle and Lemon Wall are fabulous for reef architecture and sealife. During our last stay the first week of July, we repeatedly saw Eagle Rays at Lemon Wall. You'll never have an opportunity to see these sites at Sunset House.
Second, when the north wall is rough, Divetech moves its boats to the West Bay dock. They will drive you there each morning from Cobalt Coast (all of about 10 minutes). The dive sites tend to be more northerly than those frequented by Sunset. You'll dive sites like Trinity Caves, Orange Canyon, Chain Reef and others. These, fortunately, are away from the cruise ships and the more tourist centered sites around George Town. (Once, with Sunset, our group of divers was chased off a reef by a "submarine" [two tourists in a motorized, submerged bubble operated by a diver in scuba]. Not only did he drive it into our group, he proceded to repeatedly bash it into the coral attempting to give the tourists a closer view of the fish.)
As for shore diving, Divetech has two shore diving locations; Cobalt Coast (right off the pier at the resort) and Lighthouse Point. If and when the wind and breakers make it too difficult to dive at Cobalt, Lighthouse is usually wind and wave free. When my wife and I have stayed at Cobalt Coast, the Divetech staff have gladly transported us to and from Cobalt and Lighthouse any time we wanted to go.
As for quality of the shore dives between Sunset House, Cobalt Coast and Lighthouse -- Cobalt Coast and Lighthouse have it hands down. The miniwalls are richer in the variety and health of fish, critters and coral. Granted, they don't have a bronze mermaid statue, but I'd rather see turtles (in four trips to Cobalt, I have yet to dive Lighthouse Point when I didn't see at least one turtle), eels, octopi, porcupine fish and all the rest of the critters.
My wife and I have dove with four operators on Grand Cayman, and have stayed at both Sunset House and Cobalt Coast on week-long dive trips. Sunset has a good staff. So does Divetech. Sunset's breakfasts are okay. And, if you like partying, then their bar is a lively place to be. But, I much prefer Cobalt Coast and Divetech, which is why my wife and I have stayed there on our last four trips to Grand Cayman.