I don't think you should add AF at 900p, it improves IQ and lowers performance, we can't risk that framerate falling at all. 1080 > 900 doesn't necessarily eliminate all drops either, especially if the PS4 version has drops at 1080p I'd say you would even need to go lower than 720p on XBO to get 60 FPS locked, but I'll be good.
- 1080p with normal AA and AF, 55 FPS average, no tearing (variable framerate doesn't necessarily mean tearing)
- 720p with same AA and AF (the scaling is going to fuck it all up anyway), 60.0 FPS average, no tearing.
You would choose 720p then, right? I mean, anyone can have their own opinion, but I absolutely wouldn't choose that. I can deal with a ~50FPS average if there's no tearing or weird pacing.
The example is a big extreme.
I would prefer 1080 with 55 fps over 720 with 60 ;-)
But I would also prefer 900 / 60 over 1080 which dips into the 50s