I would suggest that you actually stay over on Anguilla. It's just a short marine ferry ride away from the French side of St. Martin.
We've visited both islands several times and have always greatly preferred Anguilla.
However, if you do want and need to stay on St. Martin, I would suggest the French side. But please be aware of the fact that this is a very congested little island, traffic-wise, and that getting around by car is not real fun at times!