The clarity of the font depends very much on antialiasing and hinting. See
http://en.wikipedia.org/wiki/Hinting
Hinting is basically forcing the outline of the font into the pixel grid.
Have a look at the picture on following page
http://www.daube.ch/docu/glossary/font_hinting.html
The sanserif font in Sony Reader PRS-500 is very strongly hinted and that means it is very, very readable. It also means that any other font you load up into your reader has small chance to look nice, because it is not tailored exactly for the pixel grid of your reader.
There are quite a few threads about fonts. Some people were very happy with
PMN Caecilia 55 Roman font
http://www.myfonts.com/fonts/linotype/pmn-caecilia/ . Caecilia is not strictly serif, it belongs to the slabserif family, with serifs that are big and blocky so they can be very nicely "forced into the pixel grid" - hinted. Caecilia was designed specifically as a serif font for digital displays.
Caecilia, however, is a commercial font owned by Linotype and is quite expensive for private use. Especially if you do not know if that 30Eur purchase will look satisfactory to you.
I think that Kindle uses Caecilia.