The thing I've learned is you really can't overthink it. That plays into the psychological effect if you do. If you get a generic from your pharmacy with a prescription, odds are it is going to be the same as Propecia which has a flat dose response anyway. However, I would definitely not buy a generic online without a prescription. The only thing that I find annoying is your pharmacy can change generics at any time based on prices, which is why I had my doctor write a "Do Not Substitute" on my prescription.
If you really want piece of mind just get Propecia. That is what I will do if my pharmacy tries to change my generic.
Update: I just checked the Teva website and looked at the inactive ingredients in their generic finasteride and they are the same as Merck Propecia, just like my Camber finasteride.