Canon 400DO vs 400L???

which orute would you go, and more importantly, what have been the experiences with both\either? i would normally gravitate to the faster lens, but f/2.8 to f/4 doesn't quite make up for the 6lbs of weight (???).

I'm naieve on the DO glass... what does one get with that over the L, and with that in mind.... which lens would you go with...

thanks