Whichever you get, just make sure it is waterproof. I can't tell you how many times we've spilled something inside the bag (lid comes off sippie cup, etc.) and what saves the car/room/restaurant is the bag being waterproof. One big compartment is also better than a bunch of extra pockets on the outside, organize the stuff inside with a few smaller pouches.

If you buy a custom messenger bag from Timbuk2 (I ordered mine custom about 12 years ago) they still make it in San Francisco.