Now, let's disregard thoughts and opinions on things comfort, aesthetics and build quality, and view through purely a conceptual perspective, which console has the better "Gimmick Controller" The Wii U Gamepad, or the Switch Joy-Con? First, let's establish what these controllers are.
The Wii U Gamepad is the Wii U's main controller, with it's main hook being the 6 In. 480p touch screen in the middle, that can be used in various ways. You can play most games off the TV so long as you're in range of the console. You can view secret info and perspectives during multiplayer games that other players can't see, and you can even use both it's screen and the TV Screen for various dual screen applications, such as throwing ninja stars. Combined with it's motion sensors, mic, camera and touch controls, and in theory, the Wii U Gamepad sounds like a surefire hit, allowing for a lot of gameplay possibilities
On the other hand, the Joy-Con are a bit less outlandish, owing a lot of their design to the Wii Remote and Nunchuck combo. At home each Joy-Con rests in a dedicated controller grip. Sliding them onto the Switch console, and taking it out of the dock, let's you play in handheld mode for the vast majority of titles. When detached, you can hold each Joy-Con in one hand, or pass one to a friend for multiplayer games. The Joy-Con also feature advanced motion sensors, HD Rumble, and an IR motion camera that can detect various shapes, images, and gestures. Combined with the console's touch screen and general portability, the Joy-Con gives the Switch a lot of versatility in how games can be both controlled and presented.
Conceptually speaking, the Joy-Con are the clear winner in my eyes. The Wii U GamePad had some nice ideas, but most games didn't bother using half of what it offered, and instead just used it for Off TV play, or in the case of shooters and action games, a map. I think the main problem with the gamepad, is that the concept of dual screen gaming in a living room console setting really doesn't work. With a DS, you have both screens in your field of view, but you can only focus on one screen at a time with the Wii U, so that made games like Star Fox Zero an absolute nightmare to play, because now you have to juggle your focus between two de-synced displays in a fast-paced setting. On top of that, you can only use one gamepad at a time, and you couldn't even buy one in stores. I will give the gamepad this though, asymmetrical multiplayer is one of the better uses of it's potential, and is a concept that deserves to be explored more. The ability to play games off TV is also nice, though the range is rather limited.
The Joy-Con meanwhile is a far more appealing controller. If you're familiar with the Wii Remote and Nunchuck, you'll feel at home with the Joy-Con. Ignoring all it's tricks, it's as conventional of a controller as you want it to be. And their detachable nature allows for a lot of flexibility and comfort in gameplay, and since many Switch games allow for at least gyro controls, they actually feel useful in most games. The main thing I like about the Joy-Con though is that although it has a lot of tech, all of it feels useful. The Gamepad had so much useless garbage stuffed into it from a camera, to a sensor bar for the Wiimote, a TV button, an unused expansion port and a Mic. Almost none of it was necessary. With the Joy-Con, everything feels like it has a reason for being there. From passing a Joy-Con to a friend for Mario Kart in Tabletop mode, to feeling the texture of the different actions with HD Rumble, and even the IR camera has its purpose with Nintendo Labo. It's also a much more intuitive and less intimidating controller than the GamePad. A single Joy-Con is very easy for even people who don't play console games to grasp, while the Gamepad was a daunting and confusing nightmare for some.
TL;DR - The Joy-Con IMO, are conceptually a much better designed, and more appealing gimmick controller than the Gamepad. It's less bloated, and is far easier to understand for those who don't play a lot of games. Plus, they're perfectly adequate as a conventional controller in either handheld mode or the grip.