Wouldn't it be nicer to have a serif font on MathOverflow? As is done, say, on MSE. It seems to be much more readable, like a printed text. And I'd say making answers looking more like notes or even articles :)
There is a third option besides staying with sans serif or switching to a serif font.
This other option is making this user-definable like it is for example in the documentation for the computer algebra system GAP, see here -- clicking on Style takes one to a menu where one can select one's preferred layout settings.