I find both - as well as Adobe Digital Editions - work equally well. You can reset the default fonts easily, because I just did it yesterday. It's in a thread called "Very detailed instructions on how to change fonts," just type it into the search box. Now, unless it is a RTF book that I formatted myself, books show up in nice clean Ariel font.
Edit:
https://www.mobileread.com/forums/showthread.php?t=28447