Don't they both have trial periods? "Best" is such a subjective decision, so you could decide on the interface yourself.
One's answer would also depend somewhat on your starting point. What is the main editing program that is involved? That said, I don't think you can beat Silver Efex, particularly because it works with Photoshop smart objects. Also, remember you don't need plug-ins to do great B&W, and great B&W does not mean you must have a grainy look.