I see Wii Fit Plus as being bigger than Wii Sports Resort, and I see Wii Fit being more popular than Wii Sports. The fitness genre is probably the biggest thing to happen to gaming, ever. Largely because the fitness industry is bigger than the games industry ($14 billion in the US compared to $10 billion in the US*) and Wii Fit is disrupting it (by making it more fun and simpler).
Take a look at this chart:
http://vgchartz.com/swlaunch.php?reg1=All&game1=Wii+Sports+-+Wii®2=All&game2=Wii+Fit+-+Wii®3=All&game3=&weeks=156
Wii Fit has tracked fairly well compared to the best selling game of all time, and it didn't come bundled with a console. Now, how many people who bought Wii Sports did so for that game? I'm sure most did (especially the ones in Japan...), but I think it's reasonably likely that a fair proportion did not buy it for that game. It's less likely that a fair proportion bought Wii Fit so they would have a Balance Board for Shaun White Snowboarding (and not be interested in Wii Fit).
I don't know which will be more front loaded, but I think Wii Fit plus is a bigger deal overall, even if part of that is partially just to motivate more people to buy the original Wii Fit together with the expansion.
*Sources:
http://east.ballyfitness.com/franchising/whyFitness.asp
ESA annual report 2007
A game I'm developing with some friends:
www.xnagg.com/zombieasteroids/publish.htm
It is largely a technical exercise but feedback is appreciated.
