I run a contractors dump with drop sides and to me, if it's a contractors dump, then that's the way to go.
I've never dealt with a flatbed, but did grow up with grain trucks with removable box and they worked great.
I think if you do get the flatbed, I would try to find an old grain box and use it for your sides. If you don't have a back section for it, I would make some removable cross bars to hold the sides together better if you go tall.
Even with my truck and the contractors dump, unless it's light material like mulch, rarely does a 3.5 ton load on it actually touches or puts much pressure on the sides.