I've stayed at each 2-3 times. To me, no contest.
Cobalt Coast is physically just so much nicer and better maintained than Sunset House.
On average I've found folks friendlier and better service all around at CC. Ari is great.
CC is a little further away from stuff, but it's really not that far, nothing like being on the East End. Just nothing in walking distance so you would have to drive to anything, while you will be able to walk to some places from SH if you really want. (Once you're driving, it really depends where you want to go - CC will be closer to some things.)
I prefer the quiet and prettier area CC is in, and the cruise-ship-less view.
I prefer Divetech to Sunset Divers. In my experience better facilities, better boats, better attitudes, better dive sites. Divetech treats divers as adults, Sunset sometimes does not. Divetech does the North wall whenever they can, and the north end of the West wall if weather is a problem. Sunset Divers does the heavily dove sites on the West most of the time. Sunset House does do the south side some, which doesn't get done much by anyone and is nice.
It's a much shorter kick to stuff when shore diving at SH than CC. (Renting a scooter is good.) But the quality of what you get to is so much better at CC. And you can just go to the Turtle Reef shop nearby to shore dive - yeah you have to leave the hotel but it's worth it, it's a great shore diving site, so much better than at SH.
hmm, this has got me wanting to go back there...