Firstly, you can take a look at the recommended devices here: https://grapheneos.org/faq#recommended-devices. An important thing to note is how long both devices will be supported: 5 years after release.
I have a Pixel 6 and it works very well. I've read that many find the Pixel 7 to be very similar but an overall improvement.
GrapheneOS on the Pixel 7 is stable. There are tons of people on here who use the P7 as their daily driver. It seems they're all happy with their phones.
So they're both well liked and working fine for many in the community.
Ultimately, it's up to you, but I'd say you will probably be happy with either phone.