The Wii U gamepad is a genius idea that simply takes a lot of effort to see its benefits. Easily the better of the two, (possibly the best idea for a controller ever) but one that needs to be realized which takes time and money from developers. With this controller only one is needed, you don't need a pro controller, but even if you did the pro controller was like $50.
The joycons are wii motes that don't stay attached to my Switch, and a pro controller is a must. weird botton placement and small design means it's uncomfortable to use for one person, and a straight chore for two people. A pro controller is also $80, which SUCKS!
I only have the Joy Cons, and I find the dual-joy con approach more comfortable than any other controller I've ever had. To be honest, I don't really like any kind of pro controllers.
To each their own, but I definitely will never need a pro controller, or use one if I had it.
Anyway, a lot of people are focusing on the Wii-like capabilities of the Joy Cons and ignoring their other major application: attaching them to the tablet in order to make it into a handheld. From what I have seen, the hybrid capabilities of the Switch carry a very strong appeal for potential consumers, especially with certain types of software.
If you're a Mario Kart fan, you'll quickly find that one gamepad is not nearly enough. This is where the Switch and its hybrid mode shines, you can have up to 12 people playing at once; whereas Mario Kart on Wii U there was a smaller limit, only one person had their own screen. Of course, there's online and single layer mode, but they feel more like practice/unlock extra stuff modes than the actual game... I also strongly feel that the hybridization (thanks in part to the joycon ability to hook to the side of the console) is a large part for Mario Kart 8 being such a huge driving factor for selling the Switch. For the most part the main reason they bought a Switch after the Zelda Breath of the Wild push, was so they could play Mario Kart with us at lunch hours; and given the sales figures of Mario Kart + Switch that have been going on since the launch of the title, I can't imagine my anecdote is anywhere near a unique story.