As far as serif versus sans serif, that is obviously a user preference. However, as screens become more paperlike in the ability to display clean, crisp text, the more likely that serif fonts will work. A lot of scholarly studies were conducted in past years and decades to explore why people chose serif or sans serif. The studies showed that for printed material, which generally had fairly clean and crisp text, most people chose serif. But for computer monitors people generally chose sans serif.

The major factor was that monitors in that time rarely had more that 96 dots per inch resolution, often as low as 72 dpi for Macs, and the pitch (amount of space between dots) was fairly large. It was very noticeable pitch as you could easily discern a bunch of dots surrounded by black (or dark) space. it gave a blurry rendering of text which meant that all the little serif bits merely confused the eyes and caused eyestrain.

As viewing monitors and screens have improved, so has the usefulness of serif on them. I still had trouble with serif fonts on pre-Pearl eInk screens and most LCD screens. But Pearl eInk is crisp enough that I can read thicker serif fonts for extended periods on same. I still have trouble with serif on PC monitors and on my iPad (133 dpi low resolution) screen, but my iPod touch 4G with its 325 dpi Retina display makes almost any font readable for long periods.

EDIT: BTW, the difference between pre-Pearl and Pearl eInk screens has nothing to do with resolution as far as I can tell. The 6" screens were both 600 x 800 pixels. The difference is that pre-Pearl screens seemed to render text somewhere in between a light-gray and medium-gray, whereas Pearl screens seem to render text somewhere in between a dark-gray and black. I'm sure there are other improvements as well, but the darker text greatly enhanced the readability.