I lived on St. Kitts for a few years and traveled to Saba for vacations. No shore diving it think as there aren't any beaches - it's basically a mountain jutting out of the water, but the diving is SPECTACULAR on its own, and it blows St. Kitts out of the water when you compare the two b/c it has MPAs for one (so many more fish), and they have the most amazing pinnacles. I really loved it.
My friends dove in Dominica and liked Saba much better, but I didn't personally experience that.
Food was great. Nice restaurants. Charming island. Great hiking if you're spending any time topside. St. Kitts is bigger - more to do topside. Saba has a European feel; St. Kitts feels like the West Indies!
The diving in St. Kitts isn't bad - don't get me wrong. I love that island and can't wait to go back; but it doesn't hold a candle to Saba for the simple reason that there are no MPAs on St. Kitts, the island is extremely overfished, and topographically - Saba has pinnacles, St. Kitts has 1, and the dive ops never go there b/c it gets lousy vis b/c of the current.