the build quality, construction and finish of the marker also the trigger setup
the proto is a great marker and im not putting it down but you can tell the difference when you pick up the DM5 the extra money it costs has been well spent.
it just feels better when you use it (im sure you know what I mean) - this may be completely personal to me as far as 'feel' go's
but as for quality the Dm is far better put together.
IMO