I don't buy carrier branded phones anymore because they use their control to disable features on the phone, just to make you pay for their version, and they install games and other ad supported apps without your permission, and come with invasive, resource hogging and data collecting apps you can't uninstall.
That being said AT&T has updated my unlocked pixels like clockwork, I got the updates as soon as Google released them. I imagine the carrier version would be the same. And I think I've heard carrier pixels don't come with the same level of junk as other brands.... don't quote me on that though
$500 is a substantial savings... and you can always unlock the phone down the road if you want to try another carrier.