I am actively searching for a bag/backpack to haul my child's gear/toys/diapers through airports etc. Will have stroller and carseat so was hoping for something to "strap on" that would free up hands and might also be baby bjorn friendly. Would you recommend the med. or large cargo tote and is it comfortable to wear the strap diagonally across your chest?

The cargo tote is a good choice. We also have a market tote, which is a vertical bag which might be good because when traveling it does not stick out which is good especially in airports. Here is the market tote:
http://www.timbuk2.com/tb2/products/b...
Both have shoulder straps that you can put across you. The size depends upon what you are carrying. I think that what you have stated the medium is probably a good choice. The large may be a bit too big and cumbersome.