Define "heavy Duty"? If you are just buying this to be able to walk 100ft from your car to the office, then get whatever works for storage. If you are going to be walking around half the day with it strapped to your back then read on...
Buying a pack is like buying a shoe; you need to make sure it fits or you will be in pain with every step you take.
Do not buy online if you don't know what you're looking for! Go to a local camping store (REI or the like) and find someone who knows what they are doing to help you pick out a bag that will fit you; don't buy a bag just because it looks cool or has a million pockets and a fold out BBQ. You can look at the packs that have laptop compartments, but I would skip those and look at good 1-2 day pack that has a well built frame and hip straps.
A quick list of things to look for:
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A frame that fits your build. Most good packs come in different sizes from small to large, so you can find one that fits “just right”.
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Fully adjustable straps. A good pack will let you dial in the shoulder straps so they won’t rub or dig in; shoulder fatigue is no good. (Google some guides on backpack fitting, it will give you a better idea of what I’m talking about.)
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A hip strap that is comfortable. Hip straps take the load off your shoulders and place it at your lower core; also helps keep the pack from sliding around on your back (that can knock you off balance real quick, something you don’t want while on a ledge).
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Places to tie gear to the pack (Accessory Loops). In the off chance you need to carry a small tent or a sleeping bag with you, it’s nice to have a way to strap it down to your pack; look for them on the bottom and lower back of the pack. DO NOT buy a pack that has stupid metal crap on it like
Oakley, those things are poorly attached to the bag and will corrode after a while (assuming they stay on that long).
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(Bonus) Hydration pouch. Every pack will have a spot for water bottles, but it IS handy to be able to slide in a camel pack pouch into your bag and not have to awkwardly reach for a bottle every time you want to take a sip.
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(Bonus) Built in rain fly. There is nothing worse than wet gear; doubly so when it’s electronic, so if the pack has a built in rain fly to cover the bag and keep it nice a dry you should take that over the bag that comes in your favorite color.
The guy's at REI will help you find something that works, be ready to spend a good hour at the store trying on packs; also, make sure you try them on with some weight loaded in them.