I used to use a vest but now all I use is a sling pack. I think, it's much less bulk to have to deal with. My first pack was a fishpond lodgepole satchel, it's very nice. But I've switched and now I'm using a "non-fishing" pack, it's an Eno Possum Pack. It's small but fits all the things I need to have for a normal day on the water. I just like the fact that the pack is there when I need something, but otherwise I almost forget I've got it on me. My personal opinion, I say go with a pack.
The measurements of fly fishing: 1 part man, 1 part fish, and all God.