How does STM compare with USM? (Not in technical details, but in how it performs)
As a practical matter, STM makes for faster and quieter autofocusing. But that is only one aspect of this new lens as compared to its predecessor. The internal/rear focusing, nonrotating front element, upgraded CPU and electronics, and full manual focus are nice improvements IMHO. The closest focus distance is nearly a foot less than the previous version (2.8 ft vs 3.6 ft). Plus the filter size is the same as the previous version (58mm), so I don't need to buy new filters. And despite all these improvements, the new version weighs 15 grams less
than its predecessor (375g vs. 390g). The images it produces will tell the whole story, but on the surface, it looks like this lens was improved in every way that really matters.