When shopping for that type of item KNOW your prices very well before going.
The islands are no longer the GREAT mecca for bargains that they once were, though I think the selection and variety in such a small area is hard to beat.
St. Mart is still the better of the two, in my opinion. Not quite as many cruise ships go there, and the shops there seem more willing to bargain with you.
I don't think there's anything special shopping wise on the French side. It's generally more expensive, and pricing is often in Euros.