We have taken our DD to the parks in our Kelty backpack and have had NO problems! Our DD loved to ride in her pack, and with the added sunshade it was a breeze. She was able to see the parades without people blocking her line of site, and no one was tripping over our stroller during heavy traffic periods.
When it came to rides and leaving it with the strollers or taking it with us it varied from ride to ride. If there was room we'd collase it and bring it along (the CMs were fine with this), but if it was a snug fit vehicle we left it with the strollers. We never had any issues when it was left with the strollers. Plus our backpack has a small pack on it that we were able to throw a couple of diapers, a small box of wipes, and a sippy cup into and we were good to go, so no additonal diaper bag was needed. To us this was a huge benefit.
Here is a picture of our Kelty pack. It's a neat version as it converts to a stroller when the wheels are uncovered and the handle pulled up. It didn't add any extra weight and was still very comfortable. An added bonus was that it is tall enough to double as a highchair when none are available (esp at counter service resturnats). We would frequently park it and tour a land (ie. Fantasyland) prior to returning to pick it up. But standard cariers should collapse much better than ours did for rides and such.
If it was me, I'd take the backpack. Our DD loved her pack and it was never a problem on any of our trips that it was brought along. She was able to nap in it as she has on so many outings, he sunshade helped protect her from the sun, and the padded straps/waist belt were nice for DH and myself as it was compfy to wear. But then again, we're use to backpacking packs, so a kid was a breeze! Use what you and your little one are comfortable with, both familairlity and comfort for the two of you.