It would be more helpful if you provide what device you were actually looking to purchase. This is because there are difference between the different implementations of the processor.
But in general the Core I series CPUs have better performance and more power consumption compared to Core M CPUs. This is more important if you are using applications that require more processing power. For regular use, such as sending emails, taking notes, watching videos, etc., the hardware around the processor will be more important, like having more RAM or an SSD.