Which is better solution using ffmpeg?

I am working on web and mobile app project using ffmpeg. It's need to integrate 2 or more audio or video files. Question is, which solution is better? I'll use ffmpeg on server, or install ffmpeg on client device before using app.

I want to using ffmpeg on server. But I am thinking, it's maybe server slows down when multiple users send a request at the same time to server.