I guess the big question is, where do you plan to stay?
If you're staying anywhere from George Town to SMB, Cobalt Coast and Divetech will be much closer. In fact, if you're on SMB, Divetech will transport you to and from the boats/shops. If you want a condo, and plan to do your own cooking, then you might prefer the East end. If you want more of a hotel/inn/B&B for divers, then I would strongly recommend Cobalt Coast and Divetech.
Cobalt Coast is like a boutique hotel. It's small, and most of the rooms are nicely appointed suites with views of the shore. The restaurant is excellent, and I recommend the meal plan. Not only will it save you the hassle of finding a restaurant and going out every evening and noon, it will save you money, as well.
One big advantage that Divetech has over Ocean Frontiers (and over any other operators on the island) is the shore diving. Divetech has two shore diving locations within about a mile of each other, and each one has a complete dive shop, weights, tanks, dunk tanks and showers. If you're staying at Cobalt Coast, you have 24-hour diving. And, during their operating hours, the Divetech staff are great about taking you and your gear from Cobalt to Lighthouse and back -- whenever you're ready to go. Of course, even if you're staying at Cobalt Coast, you can still enjoy the daylight shore dives at either location.
The shore diving at both locations is excellent. I have yet to dive Lighthouse when I didn't see at least one turtle, and I often see them at Cobalt, too. The mini-wall at both is easy to get to and it's very healthy with lots of fish and critters.
Another advantage of Divetech is its proximity to both the west and north walls. One of the best dive sites in Grand Cayman is Hepp's Wall (and the shallower Hepp's Pipeline). Hepp's is so close to Cobalt's pier that you can almost throw a rock to it. Usually, you'll dive the north wall, with Divetech. The reef architecture of many of sites like Ghost Mountain and Princess Penny's Pinnacle is impressive. Divetech is also great about taking you off the beaten path, to sites like Spanish Bay Reef, where the mooring ball has been gone for more than a year and the reef is pristine.
Depending on wind direction, both the east and north walls can be too rough to dive at times. When that happens, Divetech is so close to the west wall, they just move the boats to west bay and you'll dive the northern, less frequented sites of the west wall.
If it sounds like I'm sold on Divetech and Cobalt Coast, you're absolutely right. My wife and I have enjoyed diving the Caribbean from Panama to Roatan and with four different dive operators on Grand Cayman. Everywhere we go, we end up comparing wherever we are to Cobalt Coast and Divetech. And, that's why we keep coming back.