3 of my last 4 karts have been Road Rebels and I prefer the softer seats. I tried the IMAF (loved the way the kart handled) but they were very brittle and cracked at the mounting points (barely made it 3 months and I quit using them after the second seat failed quickly). I tried a soft Tillett many years ago and liked it (I think it was a T8?). I also have used a Ribtect carbon fiber on a couple occasions and while I would argue it changes the handling, I found most of it to be corrected by simply running 1 (sometimes 0) seat strut rather than 2.
Currently I’m running a brand called Greyhound (purchased through RPM Racekarts) and it feels much like the IMAF and seems like it will be more durable.
Regardless of the seat you buy, make sure you use really large washers between the seat and the mounting points to help extend the life of the seat.