I don't know a ton about hairloss, but since I've been losing my hair for a couple of years I have tried to read up on it. Rogaine is used to regrow your hair. It is a topical solution and one person told me, "It will grow hair on a baby's butt", so I'm sure that is what you are experiencing (I'm not saying your neck is a butt). The propecia will regrow some hair, but it seeks to attack DHT, which is the primary cause of male pattern baldness.
The propecia, from what I have read, seems more designed to prevent further loss, but it will regrow hair for some individuals, especially in the crown. The rogaine will not prevent further loss, but will regrow hair for many individuals.
So, to answer your question, you should not suffer the negative side effects on propecia. There may be some others, however. Go to propecia's website to learn more.