I know both pretty well. St Martin tends to be superior in terms of convenience, beaches, restaurants, amenities, cost , rental cars and excursions.
The only advantages I see with Barbados are golf and they have beautifully decorated accommodations that is inviting and refined at the same time.
Barbados is about two to three times bigger. There are some scenic elements and historical sites but I found driving around to be annoying. Besides the lack of quality of rental cars and left sided driving, locals would block the road and try to sell you something or provide unneeded advice/directions with the expectations of getting a tip. The problem was that you never knew if they were being friendly or they were about to do something untoward as they aggressively block you in by motorcycle or other means.
There was a lot of solicitation by locals.

Overall, the beaches, waters, food and conveniences of St Martin greatly outweigh Barbados.

Rob