Fed Cts can be hell if taught by a crappy prof. I loved it at my school, and it has actually been a much more useful class than I was expecting now that I'm practicing (but I'm at a big firm; if you don't so as much in fed ct in whatever field you go into it may not be as helpful).
Commercial paper can suck too, but you said the prof is easy.
My vote would be to ask around and see what the prof is like; if people like him/her, then I think fed cts is the way to go, especially if you loved con law.