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.
This isn't about file size at all, it is about being able to see as much CSS as possible at one time.
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.
I do it too and for the same reasons. Just easier to keep your head around if you can see and scan more of the file at once. I tend to list properties with the most crucial to layout first so it's easy to find major problems (e.g., float, etc).
