One thing you should consider when deciding whether to purchase a USB scope or a standalone or benchtop scope is ease of use. We think benchtop scopes are easier to use, and that ease of use translates into higher productivity. That may not be a big deal for a hobbyist, but for a professional engineer, having a scope that's easier to use, means that you'll be more productive, and that, in turn, helps shorten development cycles and lowers development costs.
Of course, that doesn't mean that engineers should shun USB scopes. They're certainly the right choice for some applications, especially those that don't require a lot of user interface, such as automatic test equipment applications.