Different strengths. Tav's engine and defensive ability for a forward player is Premier league but he struggled a bit with decision-making when attacking. McGree is not as strong defensively but he does put in the miles. He is much more instinctive as an attacking player with, at times, a Premier league eye for a penetrating pass.
I suspect that if we had them both, Carrick may have used Tav as an inverted wide-attacker on the right. I remember watching Harry Wilson play that for Fulham and saw Tav do that really well a few times for us. He would also suit McGree's position well because he likes coming in field and he has the defensive nous to cover for Giles.
McGree was definitely bought to replace Tav when he left but it has taken Carrick to tweak the team's shape to get the best out of him. He struggled as a LCM under Wilder but some of that was, I reckon, due to the ongoing Wilder implosion.