I have dove both and enjoyed the diving. I'm sure I've written about it in some other thread.
We've done Young Island 3 times I think, and I'm getting the urge to go back sometime soon. Nice combo of a unique little resort (sort of rustic-luxe) with great service, with a diveboat picking you up at their dock for some pretty nice diving. Not big stuff, but lots of interesting small stuff and some different scenery. If you haven't seen many seahorses or frogfish yet you might get your fill here. No cattle boats, dive as long as you can with no one nagging you, at least with Dive St. Vincent. I'm not sure who's there for other dive ops right now they seem to come and go.
We did Bequia once, it's a neat little island and I didn't get to see enough of it. I'd like to go there again - assuming I can find a nice place with either A/C, or at least screens on the windows. (The place we stayed was pretty nice but at least at the time had neither, just mosquito netting over the bed and we got eaten alive at night.) Surprising variety and quality of restaurants there given the size and remoteness of the place, but it makes sense since it's a popular place with the sailing crowd so lots of biz from those folks. The diving is not better than St. Vincent and maybe not worth the extra trip just for that, but it's also good and I think the unique topside makes it worth the trip.
Can be bit of a pain to get to St. Vincent as it's just the small planes flying there, runway is too small for jets. They're supposed to be building a bigger airport which I imagine will have it's pros and cons, but if they actually do it should at least be easier to get there in a few years (Think 2010 was last I read.) We took the ferry from St Vincent to Bequia but I think it's possible to fly.