Things 3 is a very clean and well thought out application, it’s a joy to use. It’s been a while since I saw their iPad version but Mac, iPhone and Watch versions combo is one of the reasons I choose to stick with Apple’s eco system. It’s one of the few apps I’d keep if I had to make a difficult choice.
With this said, there are plenty of applications with very similar (or richer) functionality, some of them 100% free. So if money are an important factor, there are definitely alternatives. Also all of these applications are just tools. They don’t magically solve problems. If you know what you are doing, app you use hardly matters.