Some animes are made around the same time as the manga or later. Devilman anime and manga was being made around the same time and I like them both equally since they are both very different from each other. The manga is dark, violent and gritty with a strong story and great characters while the TV show is similar to Toku superheros like Ultraman and is very campy compare to the manga but still great to watch.

Tenchi Muyo was a anime first and became a manga later. I like the manga better while it's consider to be not canon since it came out later.

It's also by an entirely different author.