Like @roddyd says.
When using a VPN the only data visible is that you are connecting to a VPN. On mobile data its clear "who" is connecting to the VPN. Its "you".
On wifi its not clear "who", as many people can connect to a wifi. However if you are doing it at your own home then it can be inferred/deduced that its "you", and if you are doing it at your friend's or relatives' then it's "them" or someone they know.
So, wifi is more private.
Using vpn and wifi at a public place with a GOS phone is as good as it gets.