Personally I doubt you could tell the difference in the ride between a E250 and a E350. I keep wondering why you think a 250 would be better? Also to echo Mike about the weight, its one thing when you have just the basics in the van but do not forget about all of your camping gear and your passengers. That stuff adds up fast but they do ride smoother with more weight. Have you driven both? What about engine size and such? Did you decide to go with a cargo or passenger van? There seems to be way more E350 window vans available than E250s. Are you leaning towards an older van or a newer one? There were lots of changes made in 2008. Bigger brakes, Optional Traction Control, RSC for window vans, more body isolation, the newer front grille and fenders. The interior got a new dash and door panels and nicer seats in 2009. Other than that its just little things from year to year They really haven't changed a lot since 1992 when this body came out.
There are so many variables in ride, tires, tire pressure, shocks, springs and things that it is almost impossible to compare.
I think my E350 rides really great but its still stock 2wd except for shocks. My van even has the One Up sring option so it should be more firm. My bet is that a newer E350 would ride and handle better than an older E150 even because the suspension technology is always changing. No van is ever going to be a super soft ride but if you are unhappy with the ride you can try to improve it with shocks and such.
Here is a great forum post about 2wd lift kits you should read if you haven't.
Tires and wheel prices can vary a bunch. You want E rated tires so thats $200 plus each. Rims can start at under $100 each and go up from there.
Good luck on your search...