McTominay is instinctively driving forward; wants to shoot; be involved and will abandon his defensive duties, sometimes erroneously, to do just that.
Rice has to be coaxed and encouraged to go beyond the halfway line, and even when he does break rank, it's for a surging run into an immediate action before he studiously goes straight back to the base of midfield. He doesn't even think twice about doing so; McTominay sort of remembers his duties and then goes back, but you see often enough that he would rather a play conclude than break rank from it prematurely.
This could even make for a complementary pairing, as at base ends, they're instinctively different.
An obvious caveat is that Rice is younger than McTominay and could possibly blossom into a more concerted atacker, but at the moment they don't have much crossover in how they play or approach the game. McTominay is aggressive, somewhat impatient and will even force the play over retain the ball with little happening. Rice is safety first almost all the time and values control via retention considerably more than McTominay does.
It goes hand in hand with one being a natural CM and the other, DM, too.