I'm sorry that you are the one I have to break it down to on this one, but such is life.  Keep in mind that I am 22, that I have never been married and that I come from parents who will celebrate their 30^th anniversary in a few months after only dating 4 months before they got married. 

Fabulous. Sounds like you have already made a decision in your mind.  I am definitely not for staying in situations that can be harmful to you physically or emotionally, but don't think that this is the easy way out.  Your son will be confused and you will always compare the two men.  Maybe the sex won't be as good, or this new guy won't make decisions as well as your husband.  Whatever it is, you will never be completely satisfied.

A lecture isn't going to do you any good now, but don't mush all your problems together.  If you have a problem with your husband deal with it, but don't address it only because your "best friend" is waiting on the sideline.  Ask yourself if you would feel this bold if you knew a divorce would leave you completely alone.