When considering which Apple's S7, S8, or S9 is worth buying, we need to take into account the design, performance improvements, new features, and price of each watch. First of all, the Apple S7 watch, as an earlier model, may be slightly inferior to the subsequent S8 and S9 in some new technologies and performance. However, if the price is more advantageous and it still has the performance and design to meet your everyday needs, then the S7 is also a good choice.
Next, let's look at S8. The S8 may have some performance optimizations and new features added on the basis of the S7, such as longer battery life and a smoother operating experience. These improvements could make the S8 more attractive than the S7 in some ways. However, if you're not thrilled with the new features of the S8, or feel that the improvements aren't enough to make you spend more money, then the S8 may not be the best choice.
Finally, let's look at the latest S9. The S9, as the latest generation of Apple Watch, will undoubtedly make breakthroughs in performance, function and design. It may have a more powerful processor, more advanced sensors, richer health monitoring features, and more. If you are looking for the latest technology and the most comprehensive features, and you have the budget, then the S9 is undoubtedly the best choice to buy. With all that said, which Apple Watch is better worth buying depends on your specific needs and budget. If you don't have high performance requirements and are looking for cost performance, then the S7 may be a good choice; If you're looking for some new features and better performance, then the S8 might be for you; If you are looking for the latest technology and the most comprehensive features, and you have the budget, then the S9 is undoubtedly the best choice to buy.