Having been during Mardi Gras 2005, late May 2005 and early May 2002, I would venture a guess that Mardi Gras will be a little more crowded than early May, but the weather will be much nicer.
On a side note: This year, the crowds during Mardi Gras were SO friendly! Lots of families from New Orleans left the city and came to WDW to escape the craziness at home. I'd say they made up the majority of the crowd -- almost like Jersey week, but to a lesser degree. Many of these families brought beads along and distributed them to random people they met. DD and DS were thrilled to get some, and I was thrilled to meet so many kind people. We did not, however, go to "Party Gras" at Pleasure Island or whatever they were doing at Universal. The crowds there may have been larger and more "partying."
I don't know if as many New Orleans families will travel to WDW in 2006, however. Will there be a post-Katrina Mardi Gras for them to "escape" from? And if they do have a Mardi Gras, will those families stay home for it this year to show support for the rebuilding of the city? Lots of variables the UG probably can't predict at this point.