We often tend to use the terms "bug out bag" and "go bag" synonymously, when you get down to it, they're slightly different ways to solve similar problems.
If you actually Google "go bag", you'll actually come up with a bunch of messenger/shoulder type bags, with some smaller backpacks thrown in for good measure.
Soldiers, spec ops and especially contractors operating out of vehicles often like to have a bag of gear with them in case they need to ditch the vehicle and E&E back to safety. The size of the kit is obviously limited - it needs to fit in the nooks and crannies of a vehicle and it needs to be a manageable enough size to grab out of a burning vehicle in the middle of a gun fight and run for the hills.
A go bag's gear is usually a supplement to the operator's on-person gear--more mags, smoke grenades, IFAK stuff, comms, some spare batteries and E&E gear with snivel gear mixed in.
There's not much of a focus on wilderness survival stuff - ain't much bush crafting to do in Iraq or Afghanistan, and the cavalry will probably show up fairly quickly. Instead, the focus is on breaking contact with the enemy, evading back to safety and/or signalling for rescue.
Here's one example of what I'm talking about: NOLATAC: Operational Go Bag.
In contrast, bug out bags, as least amongst typical survivalist circles, are typically larger, multi-day packs, with a focus on food, water, shelter, fire starting and similar. They're usually more geared towards wilderness survival and heavier in weight. 72 hour kit type stuff.
Which kit is going to be more useful to you will depend on your area, plans and the particular disastrous situation you find yourself in. Like most, the best answer is probably to have both - more options.
Thoughts? Which makes more sense to you - go bag or bug out bag? Regardless of what you call it, has anyone gone down the path and built what they would call a "go bag" like we're talking here?