We are trying to make the same decision for August 2007 (Conquest vs Rhapsody). Right now Carnival has the edge because we can get a balcony for the same price as an ocean view on RCCL and the Conquest is bigger than Rhapsody. However, I think Voyager of the Seas will be in Galveston April of 08 so RCCL will have the larger ship.
I have only been on a short 3-day RCCL cruise, but based on that, I would give RCCL the edge on overall service, quality of bar drinks, and ship decor. The burgers, pizza, and buffet were better on Carnival; we thought the dining room food was better on RCCL (could be the luck of that particular trip).
The rooms are bigger on Carnival, and our room attendants have also been more personable on Carnival. Since RCCL upgraded their bedding, I would say bed comfort is comparable.
Daytime entertainment was about the same; The evening entertainment was better on Carnival.
I would sail on either line at any time; they are both good.