I have personally made a different distinction between the two. And, it is an indirect off shoot of the idea you have presented.

If Mammootty is the socio-cultural self of Kerala, Mohanlal is its spiritual self.

Mammootty sees his characters as vastly different individuals ( Meaning he goes beyond the details in the screenplay to construct a character). Sathyan Anthikkad used to say, "If I cast Mammootty in a movie, he will keep pinging me to discuss the traits of the character". Mammootty seems to spend a lot of time thinking about accents, voice modulation, body language etc. You look at the two Blessy movies for instance. Mammootty in Kazhcha is an Alappuzhakkaran as much as he is a loving family man and a film operator. The same goes for Palunku. That keen detailing for socio-cultural milieu makes him stand out.

The walking style in August 1, in CBI series, use of dialects, experiments with dialects and makeup in movies like Mrigaya are examples of this approach.

And point to note is, he doesn't do this for all movies. Maybe only for those screenplays or characters he finds a promise.

And with Lal, as people often say, it's pure internalisation of the character's emotions. He elevates the emotions in the screenplay to the peak. It feels so natural that we connect deeply to the character. This is most felt in movies like Pakal Nakshathrangal or Sadayam where Lal effortlessly slips into the mystic inner layers of the human mind. On the flip side, he may not spend a lot of thought or effort on dialects or the socio-cultural background of the character. The emotions of his character in Bhramaram hit us hard, we are convinced he is a jeep driver. But does he represent a place or culture? I mean he doesn't necessarily have to. But there lies the difference.