Strictly personal preference. Some feel softer or harder and some spin more, but for the most part, size and length have been restricted for many years by the rules.
Many golfers don't know that golf balls have to conform to the rules. If my memory serves me correctly...can not have a diameter less than 1.68 inches... can not weigh more than 1.62 oz. and it use to be, that it could not leave the club face faster than 250 feet per second. So, in these respects, most golf balls are similar, but where things can change how the ball responds, is the dimple pattern. If a golf ball did not have dimples, it would not travel near as far (about half). My understanding is, the dimples create turbulence, that ultimately increases the air pressure behind the ball and helps push it along. I also know that deeper dimples and the number of dimples affect the ball flight dramatically. I'm a firm believer, that the golfer's belief in his/her equipment, is the most important factor. I think golfers should take the time to go to the range, with 5 different 6 irons* and hit all of them for an hour. We they're done, they'll have a good idea which club felt the best (to them). This adds real confidence that you're playing the best clubs for you... you're the best judge, because you're the one going to play them. *FYI .. the stiffer the shaft you have, the more control you get! Can't tell you how many of my older students showed up with R flex or Senior flex shafts trying to get 5 more yards, but giving up direction. Again, try on the range for a long period of time so you've gotten in some good and bad shots to help determine.
Move Less ... Get Good! DUPLESSISGOLF