We stay at the SJ Marriott a couple of days every time we sail from SJ. The description from Bluefish is right on... it's a great hotel, probably the nicest on either side. To answer the other part of your question - - it is not near Old San Juan; it's on the far end of new San Juan. Don't rent a car, that will be a pain, cabs are fine. We usually walk over to Old San Juan at least once each trip - - It's not a short walk, but it's very enjoyable, maybe 30 minutes or so... don't usually have the energy to walk all the way back though, so we catch a cab back.
We took the Dawn on that itinerary a couple of years ago. We really like the Sun Class ships, they're just the right size for us, but that is really a matter of personal choice. The only way to really find out is to try it yourself.
My only observation based on what you wrote is that if you enjoyed the Disney cruise more that RCI, there's a chance you may not like Princess as much. Princess has a lot of repeaters on the Sun Class ships and would tend to be an older group than on the Magic or Voyager. A lot less action than on those ships as well. Sailing around the holidays usually brings the age down and the activities up a bit, but that week isn't really a peak holiday sailing.
Your friends are correct, Princess is a great line, we sail them all the time and love it... but as above, it's purely a matter of choice and personal preference, you need to find out for yourself.
