It doesn’t matter in a critical way (you’re not going to break anything if you do it in a different order), but you can save yourself some trouble. In my experience, the current SSMS updates almost always require a reboot after running the update. This is true for both the full install package and the update install package. If you update SSMS first, you will have to reboot before patching SQL Server. If you patch SQL Server first, you will almost always be able to update SSMS without rebooting first. Also in my experience, the reboot for the SSMS update isn’t critical. It can wait for a more convenient time to reboot if need so.
So remember: patch SQL Server first, and then SSMS.