depends on their stage of mutation, I dont know if you mean just a one time only or more than once so ill just assume 2/3 wins.
1st form: both are similar in strength thou Nemesis is faster so Round 1 would go to Nemesis
2nd form: I feel Mr.X is stronger and more brutal than Nemesis thou that tentacle could be a game changer but ill say mr X wins this one
Final form: Nemesis is a massive blob that can spit acid while Mr.X is a hulking beast like animal thing on all fours but still retains the speed, Ill say Nemesis spits enough Acid to end Mr.X but dies of his injuries shortly after