This was discussed earlier. FWIW, I actually own both and use both. It partially breaks down into: 1) do you want the process mostly preset and determined for you (Exposure), or 2) do you want more customization over the conversion and more tweaking (Silver Efex)? To throw another into the mix, I also use the old standby (no longer available), Convert to B&W Pro.
Exposure has more film types preset, and for your money, you get a really super set of color films as well. (I use the Exposure-color) on almost every thing I do. Silver Efex has the fantastic ability to apply local contrast, corrections, etc. to particular sections of an image as well overall. I converted a shot I took in Tahiti about five years ago. I had tried several stabs at it over the years, and never really liked what I got. Finally, with Silver Efex, I was able to create the B&W I saw in my mind.
They are both great products, and in a way, complimentary, rather than competing.
Nemo