It most likely depends on how good your signal is and how near you are to the WiFi source.
Don't forget you could be miles away from a cellphone mast, so that would need more power to connect to than a WiFi connection that's only metres away.
This is the correct answer.
EE network drains my battery faster than any other network because of the signal strength, my Moto X4 can go ten days standby on the O2/Vodafone networks but only 4 on the EE/Three networks. That's because the O2/Vodafone mast is about 100-200yds away, the Three mast is about 1 mile away and I've no idea where the EE masts are but signal indicates it's some distance.
Being on WiFi would help reduce the battery drain versus the EE and Three networks but it makes very little difference on the O2 and Vodafone networks.
The above is all 4G versus WiFi and it also applies to 2G versus WiFi.
3G always seems to use more than WiFi, I've no idea why.