At the risk of stating the obvious, when you’re picking a Mixed Accessory Set, the first thing to check is that it has a selection of items you’ll use. For example, a 101-piece bit set is often a great value buy, but if you don’t know when you might use half the pieces in it, then it might be more kit than you need.
You might also want to think about how the accessories in your new set are packaged; some brands supply their sets in carry cases which are compatible with their other storage options, which might be a consideration if you are looking to save space by stacking your accessories with your power tools.
It’s always worth making sure that the Accessory Set will work with the tool or tools you want to use it with, as compatibility can sometimes be an issue.