Having done both, I personally think D1's content is definitely easier - there's practically no actual maths involved beyond simple arithmetic. However, D1 was the only exam I had where I struggled to finish. Granted, I didn't do much revision for the module cos it was almost irrelevant to me but it's very hard to manage your time in the exam so you do have time at the end to check at the end. It's also hard to spot mistakes in the exam and it's hard to check over the work unless you essentially do the question again.
M1 is definitely harder and the content can be hard to get your head around at first but there was almost a moment for me where everything just clicked. M1 is more akin to the pure maths out of any of the applied modules and as such, there's almost an instinctive feeling where you know if the answer you got a as right or wrong. You don't have to be careful as you have to be for D1 but your understanding of the fundamentals behind each topic has to be solid.