m30 has shallow earcups. i gifted one a few years back. the m50 has deeper earcups and i prefer them more for that reason. regardless of the models the earcups are all on the small side.
the lineup is also not neutral. it tends towards v-shaped although the m40 is a little more neutral than the m30 and m50.
i cant say i have a/b the m30 and m40 but i have done the m30 and m50 and i could notice a difference. both do not sound bad for their price categories but my biggest issue was build quality and earcup depth which makes me prefer the m50 much more.
if you want something neutral, you likely need to look around a bit more however the m30 is not a bad pair of headphones if you like a v-shaped signature.