For me I went GM with the diesel. Better fuel economy, better ride and I could lube the frontend. The 6.5 diesel was a decent motor and I got over 500,000 miles before i sold it. The savings in fuel and such worked out fine and totally offset the extra cost of the diesel option. One major point for the diesel was I could go further than a gas unit on a tank of fuel thereby allowing me a better cruise distance (550-600 miles instead of 400-450 miles).
Next van I bought new but bought the Sprinter for a couple of reasons, first little difference in price compared to the Ford (gm new duramax not available then) as dodge would deal and Ford would not, second fuel economy was way better in the Sprinter (26mpg) and thrid loads more space and I can stand up in it and take higher skids.
Finding a used gm diesel will be very hard as the duramax hasn't been around too long in vans. The Ford 6L had all kinds of problems (according to dealer records). Used Sprinters usually have loads of miles.
For loads the gm long wheelbase is great as well as the Sprinter. The Ford has too much overhang on the rear and I've seen several with poor suspensions due to hauling a load too far back. Further the gm door opening is a little better than the Ford due to the weld edge on the Fords. Of course the Sprinter is even better as the doors fold right around to the body.
My Gm had the original tie rods when I sold it and only the ball joints were replaced over 500,000 miles, original transmission, original motor, finally rebuilt the fuel injection and replaced the fuel pump at 400,000 miles, but otherwise a cost effective vehicle. The Sprinter has also been very good for repairs so far but has required a bit more than the gm.
My view, go GM over Ford unless the Ford is a real good deal. I like diesels over gas and I work to get the mileage out of them as I hate stopping for fuel on a run and only stop for fuel, food etc in one stop (or as many needed to get there, but as few as possible).
Rob
p.s. diesel is cheaper than gas right now so big advantage in canada.