There are empirical studies out there. No reason to speculate. The results: Mostly meh (see http://news.ycombinator.com/item?id=808082). Not much of a difference either way, if any.

Looking at the history of how we write and print it is very hard for me to actually believe that serifs were put on letters with legibility in mind. One of the oldest examples of the use of serifs is this inscription: http://upload.wikimedia.org/wikipedia/commons/b/b8/002_Conra... (jpg, Trajan’s Column, see http://en.wikipedia.org/wiki/Trajan%27s_Column). And this kind of script was pretty much only used for monuments. Why optimise that for fast reading of long texts at small sizes?

I believe there were aesthetic reasons. Maybe even engineering reasons (pure speculation: it could be that carving into stone with the tools available at the time automatically produces little serifs – so they just decided to make them all look consistent). But legibility? I doubt it.

Legibility is mostly a consequence of training, i.e. habit (see the famous article by Rudy VanderLans in Emigre magazine, in the 90's). For example, the fraktur typefaces of Germany (the ones you see in nazi movies), they seem unreadable to our eyes.

For a more familiar example, programmers seem to prefer aliased or antialased fonts in their editors depending on what they are accustomed to see. When I switched from aquamacs (antialiased) to linux emacs (aliased), I found it unreadable. After some time, the reverse became true.

"First thing I tried Jost Hochuli theory that we need only upper half of the letter in order to understand the text."

And then he's comparing the top quarters of C's and O's in different fonts.

If you're going to try to determine which is easier to read, it doesn't make any sense at all to base your reasoning on how the fonts look when they're huge - you need to look at their appearance when they're rendered as they're intended to be used.

As a side note: why is it so common with negative letter spacing? Do the fonts look bad "as designed" on some common OS/browser combo? On my OSX+Firefox it just makes the text harder to read and it looks too cramped.