btc only Trezor safe 3.. 12 or 20 words ??

Hi, I’m about to buy the Bitcoin only Trezor safe 3 & I want to get the metal keep with it… but I don’t know if I need to get the 12 word or 20 word metal keep??

The Trezor website says it’s 20 word but then also says it’s 12 word on the next page I click..

Which one do I buy? Or is it optional to choose as part of setting up my Trezor. Thanks