Rockstar has announced no platform-exclusive missions, vehicles, or content of any kind. The Ultimate Edition extras are identical on PlayStation and Xbox, and the game launches on both the same day.
No confirmed performance advantage exists either. Reporting points to a 30fps target across every console including PS5 Pro, and Rockstar has not described per-platform graphics modes.
GTA VI is expected to need somewhere around 150–200GB before a day-one patch. On a Series S that is a large share of the drive, and the official expansion card is expensive. PS5 storage upgrades use standard NVMe drives and cost considerably less per terabyte.
Buy where your friends play. GTA VI launches single-player, but Rockstar is expected to add an online mode later, and cross-platform play has never been supported in GTA Online. The platform you choose now is likely the one you are stuck with for that.
Beyond that, choose on price and storage. A Series S is the cheapest route in and will run the game; a PS5 Pro is the most expensive and buys headroom rather than any confirmed GTA VI benefit. If you already own any of the four, there is no reason to upgrade for this game.