* Specific program: The rigor of a specific MA program at one university might be significantly different than an MSc program at another. The faculty, course content, assessment methods, and overall academic culture all play a role. Comparing "MA" and "MSc" broadly isn't sufficient.
* Field of study: A Master's in theoretical physics (MSc) will almost certainly be more mathematically demanding than an MA in English Literature. The difficulty is inherent in the subject matter.
* Student's background and aptitude: A student with a strong quantitative background will likely find an MSc easier than a student with a humanities background might. Conversely, a student with excellent writing and critical thinking skills might find an MA easier.
* Workload and research intensity: Some MA programs might focus heavily on research and thesis writing, making them more demanding than an MSc with a coursework-heavy curriculum. The opposite could also be true.
In short, there's no universal answer. You need to compare specific MA and MSc programs in the field you're interested in to determine which would be more challenging *for you*.