I guess my first thought would be that the Adv9 is "cooler"; if you have one you must know what you're doing <g>.
That said, and from what I know of your stuff, I think things would get cramped with the Slingshot. When "on assignment", I fit the camera (30D) with 17-85 lens, a backup (XT) another zoom (70-210) a couple primes (50, 28), lots of cards, battery packs, a charger, strap the tripod to the mounts on the bottom, put some food and a jacket in the top, a computer in the slot. It's pretty comfortable to lug around; but at Disney I leave the computer and charger in the room or car. Then I can take more snacks! I also have more room to put Disney stuff (just can't visit without picking up something). I'm talking Disney World here, so when I leave the room I usually don't return until bedtime. At DL everything is alot closer (usually stay at DL Hotel or Paradise Pier, maybe across Katella (King's Court?)) so maybe I wouldn't feel the need for the large pack. But as I don't have a small one anymore, I'd probably bring the Adv9 pack anyway.
---Ritch