maxpedition mongo is my bag. (got some tools in pocket pouch. book for reading. gloves and hat for cold days. deo-stick, small FAK, duct tape, paracord, spork, trashbag, flashlight, lighter, pen pencil sharpie and notepad, to name most.. and i still keep the main compartment empty for the stuff i might need to carry that day.. i can fit my lunchbox and soda bottle in there too.)
i might go for something larger if i would need to carry a laptop (maxped larkspur maybe?) but as i dont have to the mongo is enough.. even if it is a sling bag..
i cant get around to use a backpack.. i look goofy with one and it takes me ages to put on if its fitted to sit tight to the back when on, like they should be.. so i would likely try to carry it by the shoulder straps in my hand.. so its more convinient for me to have the one sling that i can throw around my head if i need to go little longer way than few meters.