Digital Foundry has uploaded its performance analysis for Breath of the Wild on both the Wii U and the Switch. While Digital Foundry has confirmed that this is only a preliminary test, the differences between the versions are pretty minor.
The Switch will render the game while docked at a native 900p, while the Wii U version and the Switch handheld version will run in 720p, giving the docked Switch a much crisper picture overall. Digital Foundry also compares the games audio and frame rate which appears to be pretty comparable across platforms. You can check out the full analysis below.