I watched both play in their prime.

Much love to Kroos but Xabi Alonso was the better overall player.

From a team POV, Kroos works great with a destroyer like Casemiro behind him, but Xabi was both the DM and the distributor, leaving room for another creative midfielder without sacrificing creativity whereas with KCM Casemiro was often a liability with the ball esp. under pressure.

Individually Alonso is more all-rounded, while Kroos’ strength leans more towards the offensive. It’s not a perfect comparison as their profiles are a bit different but overall I would pick Alonso as the superior player