Having enjoyed both, personally speaking I'd opt for Grenada for all the reasons already mentioned.

The falls and the rain forest are must-sees and LaSargesse, too, though it's been a while since we've been there so I may have forgotten the correct spelling. We were also able to tour the cocoa and nutmeg factories and believe it or not, this was very interesting but I'm not sure they still offer these tours.

Our trip to St. John was nice and there were a lot more physical activities - snorkeling, biking, hiking, climbing, etc., not to mention climbing the stairs at Mahoe Bay since we opted for the eco-camping there, and much as we enjoyed the former Grenada was really one of our very favorite and most restful vacations ever. We stayed on Grand Anse Beach and loved both our hotel and the beach. Good luck because both are beautiful - what a tough choice to have to make. But someone's gotta' do it, right?

Whatever you ultimately decide on, I hope you have a great trip.