I just completed building a 2 car garage/shop. I have 2 lines running from my main breaker box to the garage. Each coming from a 20A breaker. I am planning on putting 10 outlets on each line. I want plenty of outlets at my disposal. Should I use the cheaper 15A outlets and switches or should I go with the 20A versions, and why?
You should use the 15's because they're cheaper. There is no reason (code compliance reason or otherwise) to use a 20A receptacle just because you are feeding with a 20A breaker UNLESS there is only one receptacle on that circuit (such as a fridge or microwave dedicated circuit).