I voted for 'doesn't matter' because, for me, it's up to personal preference. If I was creating a site for my self, which I wanted to implement a lot of typography effect - then the main body would be a Serif font, simply because I like them a lot. However, if I was working with a client who wanted mainly Sans-Serif, then I would use it because it's what they prefer. If they were going for usability, rather than design effect, then who can blame them? It doesn't really matter and there is no 'best' among them. They both have their positives and negatives. As I said above, it's all about personal preference (as most options with design tend to be.)