You don't need the 20v. 12 or 18 will be plenty. Out of those three, I'd buy the black&decker.
True enough! I've got a bias toward B&D, due to good personal experience. Hitachi is my other favorite; they're bulletproof.
I collect coins too! My best ones are a 1919 penny and a '29 (I think) liberty dollar.
My favorite hammer is my 3.5lb hand-sledge. I call her Vera. :-D