As the owner of a roof rack system I have to agree with the LBS. It's a PITA to even take it off and put it back on the same car, much less switching between cars. With the switching factor involved I'd have to agree that the hitch option is the better one. Get one that works with the hatch of the Odyssey though, not having access to the hatch because of the design of the hitch rack can be annoying also.

And the roof rack cost me 5mpg, so they're right there also.

-R