D1 is definitely more straightforward for almost everything; there's only one topic that is challenging. That said, it's not as 'pure'/'mathsy' as M1 or the main core modules, so you might find the exam more difficult to ace since it requires some verbal answers.
Whereas, if you pick M1, it is going to be more difficult, but you know where you stand when it comes to the exams. If you've got the time, M1 is the better investment. If you have covered some mechanics in physics, you might find it to be an easy crossover.
Given that it's self-taught and that you only have two months (don't you have at least 3 months?), D1 is probably simpler. See what you think of it when you get going - you might find it really tedious.