I've had the opposite experience-- my wovens seem to fade less than my knits. I wonder if colorfastness has less to do with fabric type and more to do with fabric quality, though.
The fabric that fades the quickest for me is flannel, then some knits (although I have a couple of wovens that have faded quickly too). Even wovens that I expected would fade rapidly (with black backgrounds) have stayed looking new and fresh for well over a year. But I also have knits that still look almost brand new, too.
Sun will really fade your wovens, though. Faster than knits. When I have to sun diapers with prints, I use a folding rack thing and fold the diapers over it exactly in half, so the outer prints don't get the sun.
Karla