I picked up some Zip Lock 2-cup containers and found that they hold the same volume as my 2.7 cup lock-n-lock cups. The difference is that the latter measures volume to the brim, while the former measures volume to below below the brim to include a bit of margin so you don't spill hot soup all over yourself.
So, the Zip Locks are cheaper and lighter, while the Lock-n-Locks are more robust and give a much better seal. If I were to carry a filled container in my pack (say, rehydrating without cooking), I'd go with the Lock-n-Lock. However, that's not my eating style, so I'd probably go with the Zip Locks since they are not at a volume disadvantage.
I will add that, in my test at home, when I poured boiling water in the containers than tipped them on the side or upside down, that the Zip Locks leaked while the Lock-n-Locks did not. With cold water, neither leaked. My assumption is that the Zip Locks deform slightly when heated, allowing them to leak. The Lock-n-Locks have thicker plastic and a silicon ring in the lid, which (possibly along with the type of plastic) makes them more leak proof. So I wouldn't suggesting shaking a Zip Lock with hot water, nor putting it in your pack, but these are not the sorts of things I would do anyway.