I have canoes that have both types of seats and I have done extensive tripping using both kinds of seats.
I much prefer the flat webbed seats. I like the simplicity, breathability, drainability, the inherent cushioning and I like that a paddler can move a little side to side to adjust for a slightly off trim load. I understand the back support idea but I think that a back rest is just compensation for weak back and core muscles. The better conditioned your core muscles the less you will feel the need for a back support.