I personally like a more structured approach to building websites. But I head quite a few SEOs that claim that a flat structure is more search engine friendly. I get good results with the arborescent structure, and switching to the flat structure would mean a Google slap for a few months.

"Keyword density" is a thing of the past, but if you were to take it into consideration, then the flat structure would carry more weight, since it has less words in the URL.

There are advantages and disadvantages to both, the only way to know for sure is to experiment.