It depends on what you want from your workflow.

Lightroom is a general raw processor and image manager. It can make strong black-and-white conversions and gives you good tonal control, especially over how different colors translate to gray. If you already have a capable editor, Lightroom may not add much purely for B&W.

Silver Efex is more specialized. Its strengths are B&W-focused tools, film-style looks, presets, and selective local adjustments that make targeted tonal changes easier. That can help you get pleasing results faster and gives a workflow that feels more tuned to classic black-and-white practice.

A good way to think about it: general editors can do almost anything, but dedicated B&W software narrows that down into proven looks and controls, which can save time and help avoid less pleasing conversions.

So if you want maximum flexibility and broader photo management/editing, Lightroom makes sense. If your main goal is black-and-white styling, presets, and easier targeted adjustments, Silver Efex may be worth it.