I just got a new iMac with 256G SSD + 2T regular HD.
The data on my old computer is bigger than 256G
Not only that, but the Migration Assistant seems to want to migrate-to the SSD only.
EDIT I wasn't clear -- I know which data I want on which device, I was trying to cajole MigrationAssistant into splitting it up for me.
It turns out that the problem was my misunderstanding of "migration." I thought it was "migrate your system from old machine to new", but it's really "migrate your user-prefs & settings (mail, printers, etc.) to a new system", with an implied "all that data stuff is yours to deal with."
MigrationAssistant will move "other files" if you want, but I guess I wasn't getting that this was sort of an "oh, by the way" thing, and not its main function.