Wifi and mobile data are both based on radios that transmit data back and forth. This can present a security issue if:

1. you are transmitting sensitive data; and
2. the data is unencrypted at any point along the route

Short version: if you are using a secured website (banking, finance, medical, email, etc) it doesn't matter how you get there. The transmitted data is encrypted end-to-end:

You could have an airplane write the transmitted data in the sky and it would still be safe. You could tattoo it on your face and it would be safe. You could paint it on the side of the empire state building and it would be safe. – secessus

Longer version follows.

Websites are either secure (https:) or unsecure (http:). Traditionally, the browser indicates with a padlock icon when sites are secure. You can also click on that area to get security details such as the certifying authority, encryption type and number of bits, etc.

using wifi

using mobile data