If money or portability are your primary considerations, then get a used copy of the first edition, as it communicates the central idea (the power of visual thinking for the workplace) in an almost identical fashion and is easier to schlepp on the bus. However, if a few more dollars and a slightly-bigger book don't bother you, consider buying this new edition, as it's subtly-revised diagrams and improved explanation of key brain science concepts make it easier to understand on the first read.
For more detail on the differences between this and the previous edition, as well as a summary of the ideas discussed in the book, read on...