Take the train... it is faster. It is also less expensive.
Take the express train, not the commuter train...the commuter train stops at a couple of stations along the route.
The motor-coach is more convenient (you would have to walk a couple of blocks from the port entrance to get to the train station) and less worry (you don't have to go through the experience of buying train tickets) but if there is traffic it could take longer than 90 minutes...
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007