The 250's have TTB fronts similar to F-150's. They ride a little harsher due to the leaf springs. The 350's have a solid axle in front and stiffer springs. The 250 will ride a little better than the 350 on the front side. On the rear, both trucks are almost identical, with the 350 having an overload spring. In my experience, the rears feel about the same.
The problem with the front end is the location of the leaf shackles. Since they are located ahead of the axle, the majority of large front end movement is transferred directly to the rear of the leaf and into the chassis. If you install a shackle reversal, you can eliminate this problem and get a better ride out of both trucks.
My opinion is to go with a 350 so you get the dana 60 front and do the shackle reversal. It will ride better than a stock 250. If you want to stay stock, get the 250.