Back to the actual question at hand...
Both are good plug-ins, but handle files totally differently. I own both, and sometimes have created a B&W image in Alien Skin, for instance, then tweaked aspects in Silver Efex. One FYI -- I just upgraded to PS CS4, and have had more frequent crashes with the plug-in. Might just be my system, but I've experienced the same with almost all the Nik plug-ins under Vista/CS4. I actually like Alien Skin's presets a little better, but Silver Efex has some wonder abilities to locally work with brightness, details, etc. They are both excellent tools with realistic grain strcture, color response, etc. Alien Skin provides a greater array of "film" profiles. It for instance, has Polapan B&W, which was actually a wonderfully contrasty, grainy instant film (one I used on a number of commercial jobs years ago). I've used Silver Efex control points as a way to dodge and burn and bring out sharpness and details.
Here is a progression of images from which I created a B&W image from a color image in Lightroom, then tweaked it in Silver Efex.