I believe Stadia will default to 720p in this situation, so I'm going to assume you're using the Stadia+ plugin to modify the resolution sent to you. If I'm wrong, I apologize. The below answer still applies.
The delta between the two resolutions is so insignificant that you'll either see a little bit of aliasing from the upscaling from 720p, or a little bit of bluriness from the downscaling of 1080p. It's not ideal either way, but neither side of the spectrum will really drastically change the look of the game. Generally these resolutions are used on smaller displays, furthering the notion that it will likely not be a discernable difference regardless of what you try.