Choosing microSD storage for a Raspberry Pi
What size and speed you actually need.
Size the project
Allow space for the operating system, applications, your files and future updates. A large offline library can need much more storage than a lightweight learning project. Check the actual download size rather than assuming every collection will fit.
Read the markings
Speed markings describe different workloads. A1 and A2 relate to application performance; UHS and video speed classes relate to sustained sequential work. A high headline transfer speed alone does not describe how a card behaves as a small computer’s system disk.
Check the complete path
The card reader and host also affect performance. Buy from a traceable source, test new media before trusting it with important data and check compatibility with the device. Keep the packaging so you can identify the exact card later.
Plan for recovery
A microSD card is not a backup simply because it is removable. Store a second copy of important files elsewhere, shut the Pi down cleanly and keep a known working system image. Choose a capacity you can manage and restore, not just the biggest advertised number.
Further reading: SD Association application performance.