I've read both, and I greatly preferred "In the Plex". It seemed like a book that really had a thoughtful (if sympathetic) view into Google's inner workings. Levy's book drew from numerous interviews, and wove them together into a narrative that had structure, meaning, and was a great read.

"I'm Feeling Lucky" is a more personal book; It gets a LOT more into the nitty-gritty of personal politics, which can be interesting in a ValleyWag way, but didn't necessarily make me feel like I understood Google in a more meaningful way.

I got a lot of insight into Edwards' family, his mindset, and a much lower-level view into the Google offices.. But if you're only going to read one, read the Levy book.