You can take either on disc or on foot. It is explicitly stated in the Battalions section of the Core Rules that you can ignore any subheadings in a unit's warscroll name when comparing it to the unit entries listed in a battalion.
If this were not the case then there would lots of problems, for example Stormcast Eternals would never be able to take any Vanguard Raptors in the Justicar Battalion, because there are only warscrolls for Vanguard Raptors (with longstrike xbows) and Vanguard Raptors (with hurricane xbows).
BillyOcean's post in Tzeentch battalions was marked as the answer