Mahomes has proven year after year he can sustain an entire season and put his team in a good position come the playoffs. In 4 seasons he's 8-3 in the playoffs, hosted the AFC championship each season, never had a road playoff game, and been to the Super Bowl twice (won one).
Josh Allen hasn't been able to do that in his 4 seasons. Allen is 3-3 in the playoffs with all his wins coming in Buffalo and all his losses coming on the road.
Take last year... After the Bills beat the Chiefs in the regular season, at that point the Chiefs were 2-3 and the Bills were 4-1. Chiefs hosted and beat the Bills in the playoffs. Chiefs have also had the tougher division to work with, and have dominated it.
Mahomes, and it's not close.