Barky is better. Scheifele is no slouch in any ways, probably a top-10-ish C in the game. Contrary to some beliefs, Scheifele is very decent defensively and he plays somewhat similar game to Barkov. That's why it's relatively easy to compare them.
In my books, Barkov just simply is tad better at everything. They're both excellent all-purpose #1 centers. Can score, pass, defend, dictate the game, carry the puck, etc. etc. I feel that Barkov is just simply put bit better at all aspects (besides maybe goal-scoring) and he brings bit more value to the table.
Would be more than happy to take both on my team though.