If you intend to breed then you have to do some studying to see what best meets 'breed standards'... items such as size, confirmation would be first on my list, then temperment and finally color (unless the color is ornamental and important in the breed standard you are planning to achieve). This may not be as important if you aren't trying to achieve 'show quality' birds, but you should pay attention to the standards if you will need to sell unwanted chicks rather than have to cull them.
After having a good idea on what meets the standards then you need to try to objectively evaluate the birds for these qualities. My only exception to that would be to also give some weight on the personality of the bird and how well he treats the hens and people.
Silkies seem to be a popular breed so you may be able to find someone in your area who can help you evaluate which roo is best for your flock and plans. There is also a thread on here (actually I think there are many) that may provide a wealth of info for you to aid in your decision.
Good luck!
