I'm playing with Exposure 3 right now. My initial thoughts are it's a fundamentally different program from Silver Efex. Silver Efex is strictly a dedicated monochrome converter that takes a somewhat unique approach to monochrome conversion and offers very fine control over tone, contrast, brightness, and image structure. The selective U-point controls also add and offer a greater degree of precision, allowing the user to apply and selectively adjust certain parts of the image independent of the rest of the image. This is, in my opinion, the best tool for monochrome digital shooters who are looking for the finest amount of control and quality with their black and white conversions. It's certainly the best I've used to date.
Conversly, Exposure 3 offers the user a lot more in terms of flexability and creative looks. Working in both colour and monochrome it has a virtual mountain of different presets and simulated film types that provides the user with endless creative options, and what I would say is a more modest, typical "curves" approach to tone, brightness, and contrast adjustments. If I was a wedding and/or event photographer Exposure 3 would be an awesome tool to have. Being able to simulate and try out so many different looks quickly, and then fine tuning them to taste, is pretty cool.