Which one is more difficult, bachelor's degree or master's?

Well the question reflects the two major degree levels. And I know it is likely that both are important and difficult in their own way. However the answer is not complex at all. With all factors considered, a masters degree is definitely difficult. But it not only limits the academics and completion of the degree, it also includes non-academic factors. Here, let me take it separately so that this makes more sense:

A bachelor degree is what we can state as a starting point of your career building. You can either take it lightly or go for a concentrated field to drive all your focus there. Generally it is recommended to choose a master based on your bachelor's but we know how flexible it gets. Hence, to some extent your bachelor just bounces as a requirement for your further studies. No denying that you can start your career after a bachelor alone, but the scope for development will be minimal.

For a master's degree you need quite a lot of work on selecting the program, institute, obtaining standard test scores, finance etc. Just like I mentioned, it takes more than academics. It is a tiring process, of-course only if you are aiming for something high. In any case the curriculum and process of a masters degree is more difficult and complex. The most essential (and difficult) element is to decide your electives. It is going to take you to a particular career path.

Apart from these arguments, there is a job market that asks you for a higher qualification. And if you tackle it somehow, there are numerous others like, work experience, specific subject expertise, recognized degree etc. So ultimately, consider both important but yeah, masters do take more attention.