Should I buy a 1986 Honda shadow VT1100 C or 1985 Honda V45 Magna?

Test ride each one and see which one you like. They are two different bikes. The Honda V4 is smooth and packs more punch than a V-Twin of the same size. The Honda Shadow uses the more popular V-Twin engine and has more parts available for customization. It depends what you want and what your purpose is for your bike.