Webbed seats are more comfortable both pressure and ventilation wise. I have two 3/8 x 8 x 12 kneepads. I put one on a bucket seat and the other is under an ankle or knee sometimes or under my butt too. If it's real wavy I'm kneeling on them. They come in handy at camp too for all sort of things. Multipurpose is a virtue on a trip.

A back rest is great for having a smoke, jigging or paddling downwind with no particular place to go. Stuff like that. Not for paddling to get somewhere IMO. Your arms don't paddle they just transfer energy from your torso.