We have been to SXM 4 times and love it. We spent 2 weeks in Barbados last January and really enjoyed it. As has been said, I would say it is a bit more sophisticated than SXM, larger with more of a government presence in keeping with it's British heritage.
We stayed near the Gap but would not do so again. The noise is just too much at night. We had to shut up the apartment with the a/c on to be able to sleep. Next time we will stay near Accra beach or Oistins. Accra is just north of the gap, about a 5 minute drive but no night noise and the beach is beautiful. Oistins is south of the gap about 15 minute drive, again with a great beach.
Most beaches in Barbados have clean public washrooms and lifeguards, which is something you don't find in SXM at most of the beaches.
St. James would be about a 1/2 hour drive to the gap, you have to drive through or around Bridgetown, which is very busy.
Have fun.
