Mobile telephone data is much more consistent and reliable than Wifi, which is very much "catch as catch can".
The difference is that mobile phone
towers are put up by commercial
operators mostly covering a whole country. WiFi is short range and individual to the particular spot. In some countries, there is a WiFi data provider (like NetABord in France) which covers a number of ports
, but mostly you are stuck with individual systems to which you may or may not be able to get a connection. Note that it is not always free, and when it is not free, then you have to go through a whole rigamarole to register and pay, every time, usually, you change locations.
You can usually buy a local SIM card with data which will work all over a given country; sometimes region. If you have to choose one or the other, this is the way to go.
I have both -- a mobile phone
data router, and also a Ubiquity "Bullet" mounted on my first spreader for long range WiFi. I get 95% of all my data from mobile telephone operators.