You will feel more motion up there but I personally like the Spa deck because there are not very many cabins up there and it is close to the Gym and Spa and just a short trek down to the lido deck for food.
Crown Princess - Southern Caribbean 10/27/2007
Freedom of the Seas - Eastern Caribbean 2/24/2008
Radiance of the Seas - Hawaii Circle Cruise - Fall 2008
Grand Princess - W. Caribbean - Jan 2006
Carnival Conquest - W. Caribbean - Jan 2005
Radiance of The Seas - September 2006 - 14 nights R/T Los Angeles to Hawaii
I've been on the spa deck on Conquest and it was fine. However, the seas were very calm on that cruise so everywhere on the ship was fine. If you hit rough water you will feel it more but you can't worry about that - takes too much energy. It will also be quieter and I think you'll enjoy the proximity to most things.