Cat 4 -> Cat 5 isn't too bad (20% increase). it's changes like Cat 5 -> Cat 6 (12,000 to 20,000 min -- 66.67% increase) that hurt, e.g. Westin St John.
I'm headed to Anguilla and St Maarten in May '08 and went to WSJ in Dec '06. I doubt SXM will be more enjoyable with jr in tow than St John or even Grand Cayman.
SXM is more built up, has larger cruise crowds than St John, casinos, nightlife and nekkid beaches
(on the French side)...but plentiful restaurants and shopping. As a family, we'll be spending most of our time on Anguilla with its beautiful beaches.
St John has sublime beaches along the north shore (a short drive from WSJ) like Hawksnest, Trunk, Cinnamon, Maho and Francis Bays, and like 2/3 of the entire island is designated as a nat'l park...a terrific place to be limin' as a family
. You can get a taste of the pristine St John beaches here:
http://www.stjohnbeachguide.com/Beaches/beach_home.html
Whatever decision, I hope you report back on your SPG choice.