The performance between a 2.5 inch sata and m2 sata are the same (perhaps because of the small m2 card that supports your laptop, you won't get a big or high end ssd' but even so, you won't notice a difference on day a day usage). In your case I would get a m.2 drive and continue using the 2.5 inch hdd that came preinstalled in the laptop. So you can get the best of the two worlds: ssd speed and hdd capacity.
Regards