To me 2011 has slightly more key travel than 2015, but 2015 and M1+ seem to have about same keytravel but M1 is a lot "crisper". M1+ keyboard seems to uniformly actuate wherever you press it, whereas the shallower keycaps of 2015 keyboard mean that it feels different when you push it from corner vs center.
Maybe it’s age, but the older one feels mushy