Just Cause 3 game director Roland Lesterlin, in an interview with GamingBolt, has revealed that the open-world game will have different resolutions between PS4 and Xbox One:

The game runs at 1080p on PS4 and 900p on Xbox One. We do so much with physician and screen space effects, that we are confident our players will thoroughly enjoy themselves no matter the platform they plat the game on.

This should be no surprise given that other titles like Metal Gear Solid V and The Witcher 3 run at 900p on Xbox One as well, though other games like Halo 5: Guardians are fully 1080p. Perhaps it's just that some third-party publishers have yet to be able to reach 1080p with their titles on Xbox One. Either way, this has been sticking point for Xbox One versions of games that are available on PS4.

That said, can you really tell the difference between 900p and 1080p. I suppose if you're really looking for the difference, you can spot it, but it's not as obvious as the difference, say, between 30fps and 60fps.