They both have their strengths and weaknesses. The Barrett books structures itself like it's a scavenger hunt, so it gives you clues and then you have to figure out where the Mickeys are. You can also skip the scavenger part and just read the answers at the back of the book.
The Scollon book is "slicker" looking with more color photos. It's an "official" Disney book, but that doesn't necessarily mean it's better.
Honestly, I'd say it's a toss up...