Personally, I prefer WorldPainter specifically for terraforming. As someone relatively new to terraforming, WP is easy to learn and use, and a wide variety of custom brushes is available. I suggest trying a practice world before you import your existing world, though, to get used to how everything works. There are also tons of videos on YT authored by top notch terraformers, everything from basic tutorials to advanced tips and tricks.

Depends on your skill level, good custom brushes in world painter is decent. If you want to get super good results I would suggest you to learn how to use World Machine.