From your profile it appears you do not have status with CO or NW. As far as upgrading, there are no revenue upgrades on CO, just mileage upgrades.

I don't know what day you are flying, but in October 2008, CO will still be flying the 757-200 EWR-MCO, and while you will receive the ordinary domestic FC service, these planes do have the BusinessFirst seating arrangement with 55" of seat pitch, which would be a tremendous increase in space over the FC seat pitch on the 737-800's (38" seat pitch).

If you can score a mileage upgrade on a 752 you will be receiving far more value for your miles.

Also, keep in mind that CO economy is very cramped and uncomfortable.

As far as the Marriott, as a New Yorker, I have never stayed there, but it is certainly by far and away the most convenient, as the hotel is on the airport grounds.

P.S.: One more thing. If you want to travel in FC to MCO, the best value is probably just buying an FC ticket, which are usually very affordable on this route ($678 rt). Save your miles for another trip.