I've been to both as well and you've gotten plenty of good advice from the prior posters. I'd have a hard time calling it myself. I think the diving in Saba might have been a bit better than Dom, but not by a heck of a lot. Both are equally 'hard' to get to (Saba usually on the WinAir from St. Martin vs. the ferry, which limits baggage; Dominica has a longer overall flight time but you can connect via Am Eagle from SJ).
Saba there's definitely "less" to do than Dominica, in that its much smaller and more laid back. The hiking is all pretty much vertical. Dominica has the rivers and waterfalls and such. Neither is known for beaches, although Dominica does have some they're just a bit hidden and totally pristine/noncommericallized as yet.
As much as I would love to dive with the Mike's at Saba Deep again, I'd probably have to say if we were to go back to either (vs. somewhere new) for a vacation, we'd end up in Dominica because the wife likes a bit more than 'diving' on the menu and -- being bigger -- we feel like we "missed" more on Dominica that would warrant going back. (I know darn well we missed a lot on Saba too, but with the scale difference there's just more to miss on Dominica.) Plus we feel like we made a good friend there as well, so that's another enticement.
Both are stellar diving though. Very similar in having extremely steep pinnacle or wall volcanic formations with very healthy coral cover, although both have their shallows with busted-up coral as well. There's some subsistence fishing around both, but we found pretty good fish life. In Dom there's the chance for whale and dolphin sightings depending on time of year, and in Saba there was a chance for larger sportfish (tuna/wahoo) and shark sightings at the deep pinnacles. Neither one had particularly large cattle boat operations or extremely long boat rides to the sites - both islands emerge so steeply from the ocean floor (which gets so deep in a hurry) that the good sites aren't that far away.
Regardless of your pick for THIS year....file away the other for a future year.
