Addressing "Key Differences"
Lightroom is a digital asset manager/raw image processor primarily and one of its features is to be able to turn color images into black & white. But not just by desaturating; rather, Lightroom allows you to exercise quite a bit of control over what tones are emphasized. However, my feeling about using Lightroom for this is that while I may get what I want, it's not much like black & white photography where you used various (necessarily limited by technology) tools to achieve a specific effect. Instead of saying "I want to get incredibly dramatic skies, so I'll slap on a red or yellow filter, you just zing the targeted adjustments around for a while until you like what you see.
Silver Efex is (again, in my opinion) more closely modeling the way a photographer using film and filters would have conceived an image. There are still lots of sliders that work in the familiar digital photography model, but the concept is, "hey, here's a scene and it won't look quite right without some filtration and contrast adjustment" so you solve the problem from that angle.
Both NIK and Adobe allow you to download a free trial copy, so if you try these tools out and don't like them, no harm done. They'll just expire. If you love them, buy them. You may find you love both of them for different reasons. Good luck.