Backpack

I need a good backpack. I currently use an eddie bauer light backpack and it's super light - cost me $20.00 but doesn't hold a ton and my detector (broken down) sticks out the top. I am looking for a sleek, comfortable bag that can hold giant finds, a few bottles of water, snacks, and my detector broken down. I have been looking around at different ideas. Just wanted to get your opinions. Also - do you sit yours down in the general area and come back to it or do you carry it all the time? I find my bag slipping to the left or right while I pinpoint, and the more it has - the harder it slips. Just curious. My brother and I are currently digging old railroad right of ways and we are finding a lot of big, heavy iron, etc. Thanks!!

PS: I have a small finds pouch on my belt. This is more for snacks, water, giant finds and spare batts, etc.