I prefer the latter. It's more readable IMO. If file size is an issue, I just use a CSS compressor. Also, it's not too hard in finding selectors, most IDE's or text editors have a "Find" command to locate a selector. That's my 2 cents.
My screen is wide, my editor is wide, my code is fairly wide, but the standard format for CSS is really narrow. Every time I see the vertical format in my editor, easily 80% of the window space is completely empty, and otherwise useless, and 80% of the CSS is not on the screen and I have to page up and down, or search, to find stuff.
This is a workflow optimization that works for me. It might work for others too.