I have always liked Mustaine, even though it is clear he is a bit of a tool.
I don't think Ellefson himself would be that interesting a person, but I'm sure he would have the best story to tell, having been in the band the whole time and having seen what happened from a perspective that isn't Mustaine.
The other thing, is that I don't think Megadeth turned crap, they just changed, the same as most bands. I understand people not liking their 90s stuff, but it is still good hard rock for the most part. Mustaine is a great writer.