I've day-tripped to both and over-nighted at both and here's my thought on the two - if you can bear to tear yourself away from a day on the beach, I'd opt for Saba. It's beautiful and pristine and much as St. Maarten/St. Martin was thirty years ago. We took the Edge out of Pelican Marina (I didn't have a problem with it but many on the boat did) as a part of a packaged tour that included a minivan tour of the island with photo ops, time for shopping (which is actually fairly limited on the island) and a lunch break with a gourmet lunch included. Our van took us to The Queens' Garden Inn and lunch on their terrace was enough to tell us both we had to come back and do an over-night there. We did eventually and it, too, was quite lovely and stands out in my mind as an incredible opportunity to see something of the Caribbean besides more beaches. The island is incredibly clean and neat and well maintained and the photo ops really are awesome.
We also have done countless trips to Anguilla and love the beaches there, particularly Shoal Bay East. If you want to try another islands' beaches, these are beautiful but they are more ocean and sand albeit spectacular, both. We've gotten into the habit of giving up a few SXM days and overnighting there often during our April trip. The beaches are incredible but Saba is, too, and the vistas are spectacular.
Tough choice so good luck.