I am not on the GrapheneOS team, but as a professional, I use and recommend them both. Each have their own strengths for different purposes. It's important to have at least one backup VPN. I also recommend Mullvad, though I do disagree with the way they've handled some things recently. AFAIK GOS does not have an official VPN recommendation, but team members may have their personal preferences. We'll see what final has to say about it :)

We don't make choices on that since it requires an extremely high degree of trust to use a VPN. I am aware IVPN, Proton VPN and Mullvad have exceptional reputation but that isn't a formal recommendation. Some members go beyond and use Tor. For someone around Nostr I heard IVPN accepts Lightning and Mullvad has lightning resellers so that would likely be better for them. I just use what I like. There's a lot of things people would enjoy knowing I use, but I also use a lot of things people in a lot of spaces would lynch me for using as well. Use what is best for you and learn more about that choice to help establish trust beforehand.