I should have been more specific... When I meant "C", I was meaning both "C" and "B+". Since the vehicle at most would sleep two people (usually just me), the cabover is a nice thing to have as a place for storage, but isn't absolutely necessary.
The way to find out how stable a vehicle would be, would be to drive a fairly busy highway that has a lot of semis on it, and see which model holds its line better. This is something I truly don't know what would be more stable, as I've heard good things about the Sprinter's stability, but with a wider stance, that is a big advantage for the E-450.
I have gone to places that supposedly sell Sunseekers, but they do not carry the short "C" models (they don't mind special ordering them, but good luck getting a decent price.) Instead, they push Soleras, which are $10-20k more because the Sprinters are "cooler" (due to the Mercedes logo), and generate more profit.
This thread has brought up some interesting research, and a lot of very educated opinions. For me, I really don't care as much about the chassis, although the price difference between a Ford and a Sprinter can buy me a lot of gas.