Well think about this, let's say you have a user who can register on your site and you have a company that can register on your site. If they both have the RegisterCommand, how do you know which one you need to use? Of course if you have your namespace correct then there won't be any issue. But it's not readable anymore!
Jeffrey always talks about getting stuff clear and keeping it readable. You need to think for yourself, if I come back to my code 6 months later do I still understand what RegisterCommand means to me or is RegisterUserCommand better for me?
In my opinion I would say use the RegisterUserCommand ;)