I think that the sling packs are good for spot fishing. By that I mean, if I am going to fish brookies that day I can take what I need in the pack. Same if I am going to a pond. But, in general, I find them less functional than a vest. The slings just don't have the ability to put things (all over your vest). So, if I am going to fish Penns all day, then the Little J the next, I need ALL MY STUFF in one vest.
I have noticed that the guys over 50 and especially guys like me, over 60, tend to be vesters and the younger guys do the sling packs and chest packs.
If anyone is thinking about it, I suggest getting a decent one but not paying out the whasooo for it. Then decide on how it meets your fishing style.