imho, buy the tall one... that's the point of the sprinter for me.
I would gladly give up a mpg for the tall one. I LOVE being able to stand up inside. Getting into your riding gear.. or out of... or getting changed.. etc - it is so nice being able to stand up and stretch out in the thing.
Being able to stand up and walk around the bikes when you are tying them down.. being able to put a tall bike inside like the 950 and it fits no problem and without having to crawl around being bent over. You don't have to compress the forks or remove the windscreen on tall bikes.
The extra vertical space also provides tons of storage if you put in top cabinets like I have - all for free (no extra length).
My one upper cabinet down the driver's side is more than enough to hold all of two people's riding gear - helmets, boots, kneepads, armor, ride pants, etc - completely with space left over - and you've not used 1 inch of floor space.
...but I'm biased