I think the question "vest or pack" is kind of like which is best blondes or brunettes. They're all great.
With that said, 27 years ago, I married a fine brunette so I better say brunettes are best.
As for vests or packs, I'm a vest guy. I like lots of pockets and room to put stuff. Nothing frustrates me more while out fishing than to have to stuff a bunch of stuff in one area and then have to sort through it all knowing something is in there. With a vest, I have different types of stuff in each pocket. Leader / tippet in a pocket, my fly boxes in a pocket, nippers on a zinger, etc. I'm kind of of the mind I'd rather have a couple empty pockets than have stuff crammed in too few pockets.
Where I live in Kentucky, our summers are hot (like Tennessee) and I wear a mesh vest.
Here's the one I own and have really liked it. I've had this Fishpond mesh vest going on 3 seasons now and it has held up really well.