If you care about design and a delightful personal task experience, Superlist blows Todoist out of the water. The UI feels fresh and satisfying, and even checking off tasks is fun thanks to the little noises and swiggly lines. But if you want something stable and reliable, especially for collaboration or anything beyond solo use, Superlist just isn't there yet. It's buggy, and even basic collaboration can break down, which gets frustrating fast.
Todoist, on the other hand, is steady and does exactly what you expect for personal task management. No surprises, but also no headaches. If you just want to get things done and not worry about glitches or unfinished team features, Todoist is the safer pick.
So if you're after a beautiful, playful solo task app and can tolerate a few bugs, try Superlist. If you want something that just works, especially for personal use, go with Todoist. For collaboration, neither is a home run right now, but Todoist is less likely to leave you annoyed.