I almost forgot I had a Wii U Pro Controller. Pulled it out, and they're quite a bit different. The grip and feel are better on the Switch Pro, IMO. Main things would be the size of the Z-buttons, placement of the right analogue stick, and the size of the face buttons on the Switch Pro. Almost forgot how much I liked the Wii U Pro Controller, but the stick placement made it seem designed for FPSes, which. . .yeah.
Speaking of the button size, that's one of the things I really like about the Switch Pro Controller. But that's mostly stemming from direct comparisons to the JoyCon.