Medicine vs Aerospace engineering

Both have challenging parts. Why do you want to know?

"Harder" by what metric? Medicine is challenging because it's a very long degree, and you need to learn a huge amount of material, however I've never seen a medicine student (current or former) indicate the material itself is unduly difficult - just the sheer volume of it.

If you are good at maths and motivating yourself for a course which can often become somewhat mundane, and where it can sometimes be hard to see the forest for the trees, engineering isn't enormously difficult either. Obviously if you struggle with maths, you will struggle on an engineering degree, but most of the material isn't particularly conceptually hard to grasp, it's just a matter of practicing it.

No matter what the comparison is a bit odd because fundamentally they're very different areas, both academically and professionally. The type of content covered in each degree is wholly different, and aspects of the teaching of each degree will also be significantly different. Needless to say working as doctor in the NHS is a very different prospect to working as an engineer in industry (no call duty for the latter at least). It's really comparing apples and oranges...