Golden, I really think You should go more by what you want and if you do not mind the $600 difference, if you really want certain ports. With me, if I have never been to any of the ports, I would not mind so much and would go for the cheaper cruise, but if I had done those ports previously, I may would want to go to different ports and see them.
However, in my case, I prefer smaller ships and I would go for the radiance class ship for a southern Caribbean cruise. I am not sure if Serenade of the Seas, or Brilliance of the seas, will be doing cruises out of San Juan in August 2012 though. I know Serenade did them in Aug 2011 and Brilliance will do them in fall 2012. Not sure if there is a few months with no radiance class ship cruise from San Juan when RCI makes the switch of ships.
Another thing you may want to think about, if a cruise is not selling well, the cruiseline may drop the price about 4 months out. If you really want the best price over particular ports, you may want to wait until spring 2012 to change cruises.
I want to say too, I know credit cards say it can take 1 to 2 billing cycles to be refunded, but the one time I switched cruiselines for a cruise, I got my refund that same cycle. I guess it is "the luck of the draw." Or perhaps, it was because the cruise I had originally booked was cancelled for an extended drydock? Plus, it was the same parent company for both cruises. I had booked originally a RCI cruise and I replaced that cruise with a Celebrity cruise.