I wish to use an exercise ball as a chair. I have done this previously with several different brands of exercise ball, and it has worked for me in the past. I noticed that some balls were far comfier than others, but apart from the obvious effect of using an appropriately sized ball, I don't understand what causes one ball to be comfier than another.
What properties of an exercise ball make it more suitable for purpose as a chair? Where possible, how I can ensure a ball bought unseen (i.e. online) meets that specification?