First of all, my +1 to do the change. Summative quizzes are often done as several "one random from category" questions, and manually clicking on 10-20 categories to create a random question from each is a boring task.
On the interface: I prefer first screen since it is smaller, but titles of the buttons should be clearer like "Add random question(s) from this category" and "Add random question(s) from each subcategory of this category". Note "(s)" before question since "one" is not correct - it can be overrided by "Number of random questions" settings above. Placing a help icon near buttons (especially new one) is also recommended.
