Tractor seats are better than cane or web for sitting, but kneeling is better for the body.  When sitting, the abs, lower- and mid-back, and hip flexors work overtime to keep you upright.  When kneeling, the strong muscles - glutes, quads and hamstrings - do the work.

Of course, there's that niggling little issue of the knees and ankles, but shifting position occasionally takes care of that.

I use strap-on knee pads made for baseball players.  They cover above, below, and on the kneecap.  I don't even notice them on portages.

Dave