I'm a girl, therefore a purse I must carry. I have meds, camera gear, credit cards, and valuable trip documents that need a home (and the occasional extraneous Disney purchase). I do tend to over-carry supplies though and comfort in a bag is a big issue. Throughout the years I've tried them all; backpacks, sling bags, shoulder bags, and even the occasional tote bag (well, all but fanny-packs...I'm convinced there is a demon somewhere in hell laughing his ass off at his creation). Messenger bags tend to work best for me. They can go cross-body deflecting weight off of a single shoulder (although they do make for interesting tan lines). I like the thought of a drawstring backpack (especially since they are so light), but being a "pleasantly plump" girl, drawstrings don't fit so good over my arms.
I did carry a backpack for years, but having my stuff away from convenient reach made me incredibly nervous. The sling bag is just weird...it's like a backpack with everything on its side (not so good for any liquids you might carry).
Whatever bag you choose, the weather has to be a deciding factor as well. Certain times of the year in central Florida sees rain every single day at roughly the same time (mid to late afternoon). A very soft and comfortable bag doesn't do so good in rain. Expensive bags (especially leather or suede) can pretty much be considered ruined after a Florida rain storm (as my light purple suede LL Bean sling can attest to).
The final consideration is the bag searches you go through before entering the parks. Something with tons of pockets and dark interiors makes it harder to get through bag check. They want you to open every single little pocket, nook, and cranny.
No matter what bag I carry, I put the interior contents in ziploc or clear bags to make them waterproof and see-through for security (and it doesn't hurt when I'm trying to locate something either).
My bags have typically been Disney bags for the past several years, but this year, I'm going with a Vera Bradley Frill messenger bag. Its waterproof, a good size yet comfortable to wear (although maybe a bit heavy...time will tell), and, best of all, it's purple! Am managed to get it on sale at a Vera Bradley outlet. Since it's purple it matches my luggage and my laptop bag, and I do love to match!
Honestly, I wish I had the guts to carry my Donald or Stitch plush backpacks. Its not like I would stand out (to anyone other than myself)...there are tons of adults wearing weirder or worse. I can't bring myself to wear Mickey ears either, but I sure buy a ton of them!
Whichever bag you choose, don't worry if it doesn't work out, Disney has tons of choices when it comes to bags in the parks, so you can just get a new one! That's how I ended up with my Disney Nerds Messenger that's been with me for two trips and my Disney backpack that made it through four trips.