Flexible data and coverage
T-Mobile's nationwide 5G network on its low-band 600MHz spectrum has helped it shoot to the top in 5G available in the U.S. It has also allowed all of its customers including most MVNO users to get access for no additional cost.
Google Fi isn't fully a T-Mobile MVNO. It makes use of Sprint, T-Mobile, and U.S. Cellular for its network. You can bring two types of phones to Google Fi: Designed for Fi phones and Compatible with Fi phones. Designed for Fi phones will be able to access all of the available networks but currently, only work with 4G LTE connections. If you want to use 5G you will have to use a Compatible with Fi phone such as the Samsung Galaxy S20 5G. Compatible with Fi phones only use the T-Mobile network and will work with T-Mobile's 5G network.
While both Sprint and U.S. Cellular are also working on 5G networks, there is no compatibility with 5G network switching. Limited compatibility for Sprint and T-Mobile 5G for S20 users on Sprint and T-Mobile makes it seem likely for the future but so far nothing has been announced.
You can get an idea of how Google Fi's two network options stack up against each other on Google Fi's coverage page. To get an idea about how 5G coverage looks, check out T-Mobile's 5G coverage map, though should keep in mind that T-Mobile's map will contain 5G coverage on the 2.5GHz and mmWave network that your phone may not be compatible with.
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