The biggest difference between the train and the trolley (from what I've heard) is that you can get off the trolley for as long as you want and pick up a later one. That way you can stop and shop or take pictures or whatever and get back on later. They run about ever 15 - 20 minutes.
We took the trolley, which is a little more enclosed than the train and so not quite as nice, but we wanted to be able to come and go as we pleased. We booked it ourselves once we got off the ship and really have no idea how comparable it was in cost to the train (the trolley was $20 per adult and $10 per kid).
Another thing to consider is renting scooters, bicycles, or Harley's (on the other side of the island) and taking yourself around.
All things considered though, I wish after reading the earlier posts that I had taken the wave runner. We took our 6 and 8 yo out in Grand Caymans and they loved it (but not at 60 MPH!).
Hope this helps.