Overall we prefer St. John. St. Martin can get busy.
However, if you get out of Philipsburg, St. Martin can be a lot of fun. Friar's Beach, an out-of-the way beach with a couple down home beach bars is one of my favorite places in the world. Grand Case is also a nice place to eat, walk and shop on a small scale.
St. Martin also has the added attraction of many topless and a couple nude beaches.

Don

Think "city" vs "country"... St Martin is a very highly populated with "serious" traffic problems all over the island. St John is mostly undeveloped but has traffic congestion and crowding mostly in Cruz Bay. St Martin has a higher crime rate and many more businesses and hotels.(more like St Thomas) . St John has better beaches and has restaurants equal to St Martin's. St martin does allow nude sunbathing on Orient Beach so they have a better attitude than St John. But ..you can lie in the sun anywhy you like at your own villa on St John.

St John is American, St Martin is bi-national (Dutch/French). I suppose the best way to explain the difference is how I feel on each island. When I am on St Martin, I feel like I am still fighting traffic and and pressure. On St John (except when in Cruz Bay) I feel like I am in a slower paced paradise. They both have their appeal, but my first choice is the quiet and slower pace...