Here's the Think Tank Retrospective 50 (my latest bag purchase), with the gear that fits inside. Great bag when you need to haul 2 bodies and lots of lenses, etc.
I didn't really know what would work best for me when i got my pack (lowepro flipside 400), But i knew i wanted to be able to carry the 150-600 mounted. I can do that and a couple lenses. But,,,then i got another body and can't fit it with a lens mounted and everything else. It came with a small bag so i carry both bags at times.
I have a Big Bag (a Manfrotto Pro Backpack 50) that can carry everything, and that bag pretty much lives in the trunk of my car. I also have some smaller and medium sized bags for those times when I need to carry a small amount of extra gear but, typically, and unless I have strong reasons to think I'll need a lot of extra stuff, I prefer to travel as light as possible.
Thanks for all the comments. I've decided for now to use my current lowepro event messenger and my old smaller event messenger to see how that works out. I did look at a number of backpack options last week at the camera store so one of those is definitely a possibility in the future.