I assume you're comparing the SansAmp BassDriver DI stompbox to the rackmount version. I haven't used the rackmount myself, but I'd assume you'd have more control over mids (since it has a mids knob), if you care about that sort of thing.
The funniest thing to me in the manual for the stompbox was where they say something like "to adjust mids, simply turn up or down the treble and bass knobs accordingly."
I love the sound of the stompbox, and most modern active basses have enough EQ controls onboard to get your mids into shape before you hit the DI stage anyway. However, if I was making a long-term purchase for my studio I'd go for the rackmount, it's worth the slightly extra money for rackability and an extra knob IMO (even if you don't use it much.) I plan to purchase one this year.