If you’re looking to get college credit, I’d say your chances are stronger with U.S. History over Comparative Government. If credit doesn’t matter as much to you, or if both will/will not confer credit at your choice colleges, take Comparative Government if that’s what speaks more to you! I can’t vouch for C.S., but some at my school consider Principles to be an “easy” A.P. class.
As for AP Bio, I learned more in the class than in any other class to date in high school. I’m not going into science, but I enjoyed the class and how much I learned. It is indeed rigorous, and the A.P. exam tends to be one of the more difficult ones. Again, if testing/credit is not important to you, take what speaks to you. If credit does matter, A.P. Bio is a reliable exam for credit purposes. Good luck!