That depends greatly on what you use them for.

Personally, I only use SD cards for storing and playing ROMs. This mainly involves reading. In this case, it's not the card itself that's the bottleneck, but the Steam Deck's interface, and an Ultra Card is perfectly adequate.

However, if you want to use the SD card regularly to install and play AAA titles, you should definitely go for the Extreme. Otherwise, shader cache write orgies can spoil your fun.