Either should be fine. Pixel 6 was released a little over a year ago, so it has 4 years left of support. Pixel 7 has 5 years.
The GrapheneOS build for Pixel 7 is no longer considered "experimental" and many on this forum are using Pixel 7s with GrapheneOS as their daily driver.
By all accounts, the two models are pretty much the same, with some improvements in the Pixel 7. I personally have a Pixel 6 and haven't had any problems with it.
Pixel 7 is 64-bit only. There aren't many 32-bit apps out now. Although I haven't actually checked, WhatsApp and Instagram are sure to be 64-bit. If you got a Pixel 7, the only apps that wouldn't work would be super old apps that haven't been updated in a very long time.
Ultimately, it's up to you which one to get. If I were buying a new phone now I'd get the 7 because it's newer, has some hardware upgrades/improvements, and will be supported for a year longer than the older version.