You might have better luck getting an answer if you posted on the Princess, Carnival, or Norwegian threads.
Hopefully someone can help you out with some comparisons of the lines.
Sorry, I can only compare Celebrity, Princess and a line that no longer exists. Otherwise, I'd help you out.
Good luck, though, getting someone out there to respond.
Regardless, enjoy your Mex Riv cruise. I am booked on the Mercury December 2006!