This article will help you sort out when you should use "data" (or "cellular data") for Internet on your smartphone and when you should use "Wi-Fi" for Internet on your smartphone. Let's start with definitions or descriptions of data and Wi-Fi.
Cellular data means using the technology on your smartphone to connect to a nearby cell tower to send and receive data. Cellular data has a pretty good range -- 10 or more miles, depending on terrain and structures. Cellular data is the type of Internet you see on the advertisements for "4G" and "5G". This refers to which generation (4th or 5th) your smartphone is connecting. Your speed, when using cellular/data, depends on your phone's capability, the tower's capability, your signal strength to the tower, and the number of other people also connecting to that tower.
Wi-Fi is a short-range wireless Internet from a fixed Internet connection at a home or a business. Wi-Fi is what you may have at home or what you find at a hotel or coffee shop. You have to be in the building or on the property to get a signal as it only works within about 150 feet of the Wi-Fi router. Wi-Fi is the type of Internet that you must first "choose" in your settings before you can connect, and in many cases, you must enter a password for access. Your speed, when using Wi-Fi, also varies, based on the speed of the Wi-Fi connection you are using, your signal strength to that Wi-Fi connection, and the number of other devices sharing that Wi-Fi connection. Due to the number of factors involved with either type of connection, we cannot say that one is always better or worse than the other in terms of speed or reliability.
So, the question is which one should you use and when??? Well, that depends.
Smartphone plans include cellular data. How MUCH is included depends on your package with your provider. If you have a limited amount of data (gigabytes sent and received) with your package, then you will need to watch how much you use your Internet off cellular towers so your speed isn't reduced or (worse yet) you aren't charged extra. Activities like streaming video (movies, tv shows, Facebook/Tik Tok videos) use a lot more data per minute than activities like sending and receiving E-Mail or simply looking up phone numbers or scores. Any Internet used over cellular data will count against your allowed data plan. Even if you have an "unlimited" data plan, your speed could be reduced when you hit a certain limit. As we mentioned, the speed over a cellular data connection varies but with a strong signal, your speed should be fine for most Internet activities.
The data/signal is encrypted between your phone and the tower, so it is considered safe to transmit personal/private data over a cellular connection. The biggest issue with Internet over cellular is typically the data limit mentioned above. The second biggest issue may be speed. While the speed over cellular is typically fine for most things, it may or may not be adequate for live streaming.
Internet over Wi-Fi comes from a local land-based Internet connection that is broadcasting a Wi-Fi signal. Like cellular Internet, speed is dependent upon several factors, including speed of the original connection, Wi-Fi signal, interference, and other devices connected to the Wi-Fi. Home Wi-Fi is generally faster and more reliable as you have more control over the network. Wi-Fi from a restaurant, hotel, coffee shop, library, or other business/public space is rarely very fast, even when you do have a strong signal. While some businesses need to keep their connected customers happy, for many, it's just a "free" convenience they offer and they don't put a lot of effort into creating a robust connection. Also, some of those connected customers could be hogging more than their fair share of bandwidth, leaving less for the rest.
Public Wi-Fi is a huge risk for sending private data including financial, personal, or medical information. There is no way for you to tell if someone is tapping into the data of that public Wi-Fi signal and capturing it for ill purposes. Even when you had to provide a password for the Wi-Fi, if it is not under your control, you should not send or receive any private or sensitive information.
Bottom Line:
If you're at home or at a trusted location, use the Wi-Fi. It will save your data and might be faster and more reliable.
If you are not at home or at a trusted location, and you are not doing anything that requires sending sensitive/private information, go ahead and use an available "public" Wi-Fi connection. This works great for catching up on social media, reading the news, browsing websites, or streaming videos. If you have the data to spare, go ahead and use your cellular data --- or whichever between cellular data and Wi-Fi performs the best.
If you are sending sensitive/private information, make sure to disconnect from a public/shared Wi-Fi connection and switch to cellular data OR wait until you are on a Wi-Fi connection you can trust.