Heinlein is one of the greatest sci-fi writers ever. I have read both books. The Moon Is A Harsh Mistress was one of my favorite books in high school. It's not very long and of the 2 that's the one I would read to find the MULE references. Time Enough For Love was written when he was older (and generally regarded as being slightly crazier) It is a much longer book and I forget a lot of the plot now because it has been a lot of years, but i don't think there has ever been a bad Heinlein book. He is such a great writer that even though his books are from the 1940's - 1980's and his view of the future was wrong in a lot of places they still hold up really well.
Thank you for your reply, dynadan.