St Vincent diving is very nice, there's variety in the sites and lots of interesting critters to be found. We've been there at least 3 times land-based and once on a liveaboard. I think there's some nice sightseeing and hiking to be had, but we've mostly only seen the island from the water because we're either diving or too content in our nice resort to leave it. But it's sort of a pain to get to, they've got a lousy runway, no jets and small planes with luggage restrictions. Used to be somewhat better air service but AFAIK currently the only scheduled air is LIAT which doesn't exactly have a stellar rep (and is one of the only 2 times we have lost baggage in many dive trips - only found due to the persistence of the folks at Young Island resort - so I'm extra biased.) If they ever finish (start?) the new airport we'd be there more, though I suppose it will change the place.
(Bequia is a really neat little island with some more good diving, harder to get to than St Vincent even, but a good add on if you're already there. Ferry runs regularly between the islands. I'm not sure I would do both in only a week, but keep it in mind if you like places less visited. It's very popular with the sailing crowd, so it's sort of off the beaten path, but with restaurants.)
Grenada is a very interesting island to see and not too bad to get to. I'm thinking of going back there. But we only did a couple dives there from a liveaboard, and it wasn't enough to form a opinion on the diving there in general.
I've always heard pretty good things about the diving in Dominica, and less good things about getting there. I've never really considered Statia as it doesn't sound like there's enough of interest to make it suitable for a dive intensive (3-4-5/day) trip, and even 2 a day for a week seems like it might be a stretch. But I haven't been to either place so take all that for what it's worth. There's certainly been occasional mentions of both places on this board, if you haven't found them yet.