At the time of its release, the manner on how it they’d marketed that was confusing as they showcased the Wii U gamepad as if its a portable console (its not) since its rather the main controller for the Wii U console. I was in high school when the console was first revealed, my reaction to that was “WTF?” as I thought the gamepad was portable, as the ads often flex that it can be (or the way they showed it came off as that).
3DS was a new system and significantly more powerful than DS.
You can see it mostly on what 3D games on DS look like in comparison, they were generally very limited. Of course 3DS had a rather low resolution (though already higher than DS), but it could do a lot more. Roughly, imagine going from N64 to Gamecube or Wii visually.
3DS also had a few specific abilities, obviously stereoscopic 3D, but also an analog stick (of sort, but still making a huge difference), console to console passive communication (street pass), NFC reader, and motion sensing (surprisingly great for aiming, made shooting in Zelda games a breeze).
3DS actually had its own new model with the “new 3DS” (that name… they never learn).
This one was mostly the same but more convenient. A lot more stable 3D, especially if you’re combining it with motion which was basically impossible before, a second (rather shit) analog stick, a second pair of triggers. You could have these new inputs on a standard 3DS through an accessory, so they just integrated them.
New 3DS was also more powerful, but a game needed to be specifically made or updated for the n3DS to access that extra power. Very few games were updated, and even fewer were exclusive to new, only notable one probably being Xenoblade Chronicles.