You're looking at a 480p resolution on a low quality screen with the WiiU gamepad, compared to a 720p high quality screen with the Switch. Colours and visuals will look much better, with the Switch.
In addition, impressions from Switch events have said that BotW actually runs slightly smoother in handheld mode on the Switch, compared to docked. Apparently running at 720p, and not 900p when docked means fewer frame drops. However, those impressions were from a non final build, so that might not be the case when it's released.