Bertoia side chair: seat cushion upholstery?  

Neither.
I really like the felt seatpads and backpads made by Parkhaus in Germany.
For the Bertoia side chair, they make a two-piece set that attractively exposes some of the wire on the seatback.
Catalog linked below:
http://www.parkhausberlin.de/collection/suit/PARKHAUS_Suit.pdf

Thanks fastwd
I hadn't seen those before. I like their selection, thanks for sharing.
I prefer the simplicity and look of the regular seat cushion though. As I won't be sitting in this chair very often, it should provide enough comfort.

Gustaf
I have a pair of side chairs in yellow and the Knoll yellow vinyl seat cushions make a world of difference in terms of comfort. They are perfectly appropriate and swell; you don't need leather...the vinyl seat cushions look high-end. (Make sure you get real Knoll cushions; the cheap copies look and feel cheap!)

Thanks Barry!
That is good to hear. I am leaning heavily towards the vinyl, actually.
All this is of course only because someone mentioned the Knoll sale...