Until now, you've had to buy a Seagate card if you wanted to augment the fast internal storage of your Xbox Series X or S. Now, however, you have a slightly more affordable option. Western Digital has released a WD Black C50 expansion card line that starts at $80 for a 500GB model. That's $10 less than the recently price-cut Seagate equivalent, and easier to justify if you only need room for a few extra games. A 1TB card sells for the same $150 as its comparably-sized rival.
You'll need one of these proprietary add-ons if you want to add space for ready-to-play Xbox Series X/S games, which require solid-state drive cards that are as quick as the SSD built into the console. Conventional external USB drives can only be used to archive games or play Xbox One titles. That's fine if you only have a handful of active games at a time, but you'll want an Xbox-native card if you want access to many titles at short notice.
These cards are still costlier than the NVMe SSDs you can buy for a PlayStation 5. However, the price gap is now small enough that the storage format probably won't affect your choice of system. It's only an issue if you want 2TB of additional capacity. Western Digital doesn't offer a 2TB C50 card, and Seagate's $280 Xbox option is decidedly more expensive than PS5 options like Sabrent's $160 NVMe drive.
WD Black C50 ExpansionCard for Xbox$80