well it depends on how much cheaper
the 8MB cache is used by the HDD hardware itself, so its irrelevant to the xbox, meaning access times will be faster.
BUT, not THAT much faster....
I got my WD 120GB HDD with 8MB cache cuz it was only $15 more than the maxtor 120GB HDD with 2MB cache.
its really up to you
look into lockable/unlockable drives though, you might want to lock it.