Both are decent concepts, but the joy-cons are executed far, far better.  

The joy-cons fit perfectly with the concept of what the Switch is.  They allow you to play in handheld mode or make your own type of Pro controller or the more free-hand dual joy-con approach.  Also for a lot of multiplayer games they count as 2 controllers.  And of course they have motion controls for Wii style games.  The only thing I don't like about the joy-cons is the HD rumble, which I feel is an unnecessary feature that drives up the price.  Other than that the joy-cons are an awesome concept with a great execution.

The Wii U gamepad to me is a bunch of wasted potential.  But to me the wasted potential is in the Virtual Console/eshop.  First of all they could have put a ton of popular mobile games on the eShop.  I mean if I have this touchpad, then why not give me touchpad games?  They missed the whole mobile market which could have been there.  On top of that they could have put a ton of DS and 3DS games on the Virtual Console.  You can use the TV to be the "top" screen and the gamepad to be the "bottom" screen.  I would have liked to play a lot of those DS/3DS games on the big screen.  At first it seems like DS and 3DS games would have to be handheld only, but it wouldn't have been hard to make it happen on the Wii U.

So, basically I can't fault the hardware designers at all in either case.  But whoever was thinking about the Wii U didn't really make it nearly as consumer friendly as it could have been.

The dual screens worked with DS/3DS because you could actually look at the two at the same time. That isn’t the case with Wii U, you’re either looking at the TV or looking at the Gamepad. So 3DS/DS style 2-screen games do not work on Wii U at all. Not to mention the two screen play completely defeats the purpose of off TV play... if that purpose wasn’t already defeated by the fact that it was like a 5-8 meter range.

The stuff that the gamepad did do as a two screen device works no more efficiently than going into another menu screen on a single screen device. Games like EBA can’t be done on Wii U the way they can on DS... not unless they fit it all on one screen.

One sort of game that could conceivably be done better is a game where a person has a screen hidden from other players, and they compete against each other... and the Switch can do that way better since you can have multiple people with their own hidden screens.

It's true that not every game on DS/3DS would transfer well.  But that ignores the fact that a lot of games have the top screen as the primary screen, and the second screen can often be things like inventory or map or something else that you only look at occasionally.  Really most of the games that I've seen on the DS/3DS are like that.  For example I would really like to play New Super Mario Bros on a big screen.

I would say games did that as more of a “because we should use the screen for something” thing rather than something essential to the game. Even the way XCX used it (and was praised for using the Gamepad correctly) would be transferable to another menu screen. I don’t see much convenience gained by simply looking at another screen rather than simply pressing the select button.

There is one benefit to the Gamepad over the joy cons as far as I can see, and that’s that it has a touchscreen interface when in console mode. The Switch can have that too in handheld mode, and in console mode it can use the gyroscopic pointer (which works much like the Wii IR) it simple directional controls. When it comes to applications that benefit from this, they’re not going to be very action oriented of attention is required between two screens - mostly things like Miiverse and Netflix; IMO the Apple TV remote is superior to the Gamepad for this sort of thing since you don’t need to look away from the TV screen to control what’s happening. Future joy cons will have trackpads if they’re important for any of Nintendo’s applications - right now I don’t think games generally benefit from these: maybe Civ 6?