I've been to Harbor Island the the Rockhouse. Candidly, I would put it in the "liked it, but did not love it" category. Rockhouse had very friendly service and nice people, but was frankly better in the web photos than up close. Some things a bit frayed around the edges. It was also loud when we were there. The restaurant/pool/bar was a pretty popular spot at night and the collective noise was an issue. I've stayed at Pink Sands in a prior trip. I thought that was the better option as its right on the beach. In fairness its been two years since I have been there and some things may have changed.
The beach is amazing. Don't expect the shopping and dining options to be like what you see in SBH either. Its always dangerous to compare places, but I found the collective "vibe" to be more Anguilla than SBH. Hope that helps.