Not enough is known to be able to compare them apples to apples. They are both antiandrogens and both have not completed fda trials (CB is still going through them and RU never finished all phases).
No one truly knows which one is better or safer. As with any experimental treatment you are taking a risk and it is up to you to decide how much risk you are willing to take on in order to battle male pattern baldness.
I am using both at the moment. I alternate RU one day and CB the next. It's still too early to tell how well they are working and I'll never know which one is working better but I hope for the best for my hair.
Good luck on your journey.