I can give you some pros and cons, but you need to decide what is important to you based on your ultimate goal from wanting to learn an accounting application.
Pros- The application you are most likely to run into especially if you ever work with small businesses (they have something like 70-80% market share of small business accounting)
Cons-In an effort to make it user-friendly, it is actually not a good platform to learn accounting due to complexity. There can be 3-4 different ways to accomplish the same task in QB.
Pros-one of the leading cloud-based applications which in my opinion is definitely where almost all accounting applications are headed.
Cons-while not as complex as QB it still has a lot of functions that learning may or may not translate to using another accounting application. It can also be expensive to get a full set of functionality.
Sage isn’t a bad option, it is more cumbersome and less popular than the other two programs, so if it were me, I wouldn’t learn it unless you absolutely have to down the road.
Final thought- all accounting applications will perform similar functions but the process they follow can vary greatly, so learning one will not necessarily transfer to knowing how to use another. If your real goal is to learn the mechanics of accounting then gain an understanding of the theory behind double-entry bookkeeping rather than learning how to operate some specific software.