I prefer Serif fonts, I think they kind of look more natural (maybe it's because most books are written in serif fonts). On backlit materials (screen/lcd), I find sans-serif fonts more clear and easy to read. On non-backlit materials (ebook reader screen - paper), I still prefer serif fonts. It's difficult to explain why, i kind of find that words in serif have more impact, more weight (yes i'm not really clear, but i tried
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But after that, to choose beetween two kind of serif (or sans serif) font is too difficult for me ...