Edit: Also the icons are much better in the second one. The first one has all monochrome icons which makes them much less useful for quickly scanning for the folder that you are looking for.
I can maybe agree that the first one looks more "polished" but at the same time I actually feel a but uneasy looking at it because my brain has to work harder to discern the UI elements due to the lack of colors and low contrast. Although the lines are heavier in the second one, I find it much easier to filter out the data that I am looking for.
Also the first one wastes screen real estate like crazy. I can only see 7 emails in that one, whereas the second one shows 14. I want all the emails that I receive within a single day to be able fit on a single screen so I don't have to scroll. Frankly, I think the first one sucks.