I have a pack by Kelty that very similar to the illusion. i relay like my kelty, and when i brought it to europe it was nice to have both a top and front openning like the illusion so you don't have to dig around everything. also, the illusion can get bigger or smaller as you fill it or unpack it and it fits more than the other one. I'd say go for the illusion, just because it's a more traditional "backpacking pack," it can hold more, and it's adjustable.
Plus, i like all the little extra pockets and loops. you can put guidebooks and other easily reachable things like sunglasses, etc in the pockets, and if you go hiking someday, you can attach a sleeping bag to the outside loops if needed.
We are not here for ourselves alone