Yes, MOBI is limited to 3 fonts.
Epub is not.
PDF only works if you make the papersize very small, like a quarter of lettertype or a quarter of A4 maximum.
Even at half the size of A4 or lettertype the text is barely readable, and much finer than mobi...
I've never actually tried reading PDF's on my jetbook, other than reflowed.
For reflow the papersize does not matter, but if you want to see it fullsize (which I doubt the jb can do (or the aluratek) it does.
I remember that I could increase the fontsize on most documents though (epub/mobi/pdf reflow/txt)! If you want to read better, perhaps increase text size.?