Had a Wii U and was quite pleased with it.

The 3DS struck me a bit too much as "more of the same" of the DS family, and I wasn't really interested in most of the games that came out the first few years. Subsequently I never bought in to it, despite probably considering the DS family my favorite game systems ever.

It's my impression that most of what was good on the 3DS has yet to be ported to Switch.
Whereas the Wii U's library has been almost completely compromised.

But maybe this isn't your specific concern?

Both, but I may as well have had only a 3DS.

I didn't really play anything on Wii U except Wii Fit, Earthbound, Fire Emblem (GBA), and Xenoblade Chronicles X. And I do hope every single one of those games comes to Switch - although Ring Fit kind of replaces Wii Fit for me.

Both for me as well. I bought my 3DS shortly after they lowered the price, and later bought 3DS XL and then finally a New 3DS (non-XL). PS4 was the first console I owned in the gen, and I didn't get a Wii U until late in its lifecycle. The Wii U had some great games, but it never appealed to me much as a piece of tech. I thought the controller was big and clunky and would have rather had that cost invested into a better console. That controller is what kept the Wii U from getting price cuts.

had both.... wii u is still the best remote for my tv :P

I presently own both a Wii U and a 3DS but I got both of them AFTER I got my Switch. Prior to the Wii U and original 3DS, I had owned every Nintendo console and handheld except for the VirtualBoy. I loved and still do love my Wii and DS. When the Wii U and 3DS came out, I just didn't understand them. To me, the Wii U was just a clumsy, expensive Wii with upgraded specs. And the 3D technology in the 3DS was totally lacking. So I continued to just play on my old systems. Prior to the Switch launch, it was clear that Nintendo was on the right track again. The Switch launch did not disappoint and I got one during the first year, while it still lacked top-quality games. Since getting my Switch, and absolutely loving it, I got the New 3DS and Wii U for the following reasons.

One big reason that I got both the Wii U and New 3DS is because my old Wii and DS are physically aging and may not last much longer. My Wii has been sent to Nintendo twice for repairs and now Nintendo has discontinued repairs for the Wii. The hinge on my DS is so loose that I'm afraid it might split into two at any point. The Wii U plays Wii games and the 3DS plays DS games so I get to play all my favorite old games on newer, more reliable systems. *

Another reason I got both systems is the crazy good library of games. I got both systems at the end of their life so that opened up the option of playing lots of fun games I had never experienced like the Mario Kart 7 and the 3D Mario games. Furthermore, these old games are really cheap now on eBay and other places.

Over the past several years, TVs have advanced a lot and 4K is now the norm. While this has happened, old non-HDMI connections have gone mostly extinct. With my new razzle dazzle TV, my Wii still connects but the picture is often (randomly) distorted and always darker than it should be. The Wii U allows me to play my Wii games through an HDMI connection which, as a bonus, allows me to benefit from upscaling. It's pretty nice!

Finally, another reason I got the New 3DS is because they finally got the 3D technology right. Playing Street Fighter, Metroid, and Zelda in 3D with the new eye-tracking 3DS is surprisingly fun, and you can't get that on any other system. Incidentally, the 3DS Zelda (A Link Between Worlds) is now my favorite Zelda game and I've played ten of them.

* The ages of my Wii and DS have become my biggest motivator for getting their successors. I recently lost my son who grew up playing the Wii and DS with me and his sister. I can't bring my son back but I can relive good memories of him by playing old games on new, healthier systems.

^^ Might I recommend the old Mega Drive games for 3DS?
The parallax scrolling is absolutely stunning in 3D.

I already played the few 3DS games that really interested me, as well as everything on Wii U I consider worthwhile.

Had I skipped both systems I really wouldn't have missed out on much by going straight to Switch.

I’ve had 3DS only. By the time I’ve decided to buy WiiU (it was early 2015), Nintendo already announced that they are working on NX, so I’ve ended up not buying WiiU after that.

I already commented but since everybody is sharing when they bought it I'm now doing it...
I first bought my 3DS after 6 years of non gaming in August 2013 with Animal Crossing and just last year I bought a NN3DS. I bought my Wii U in June 2014 after I won a Mario Kart 7 tournament in my small city and the prize was a copy of Mario Kart 8 so I had to buy the Wii U (I was already planning to buy it), maybe that's one of the reasons I liked more those consoles than the Switch, I have better memories of them when I got them than the Switch.