IIRC,
The 250 has 2" blocks, the 350 has 3"
you can get the same spring pack on the 250 as the 350 (ie...my 250 has the overload spring)
the front and rear axles are the same in the 250 and 350, except for the rear axle in a dually 350, obviously. starting 2002 they went to a "true" dana 60 up front
you can figure out the spring packs from the codes on the tag on the door.
Get a 250 crew cab short bed, unless you're concerned with resale value. When I bought my truck, I looked for 350s and 250s. The main factor for me ended up being distance to pick up the truck.
there's a good chance i remembered some of this wrong, but my main point is that when i researched it, I came to the conclusion that they're very much the same truck.
hope this helps
Paul