If we are talking about 3DS why is this in the switch section? Also 3DS and Switch are two separate systems each with their own systems, not to mention one has Homebrew backs and mods while the other is JUST BRAND NEW. Is not fair to compare something that is not within the same age.
To answer the question, yes i use my 3ds and switch, i only have 3 games for switch (2 if not counting the port of shovel knight. Should probably regret buying for $25) While i have more 3DS games and flexible things to do. Though i can't wait soon enough for Xenoblade chronicles 2 to get here, i hope some good stuff come along the way.
The switch still has things 3DS doesn't like 2 screens which is useful depending on game, and offer different play styles. Switch has a touch screen but no stylus, having finger prints all over isn't really something i like, nor can i accurately use over a stylus. Even still i don't like stylus either so is just a matter of adapting. 3DS also has micro phone, camera and 3DS. Switch has 720P resolution (Portable speaking not docking)
In the end, they both have totally different things developers can do with it. So why not use them both? In the case of "Which game i should get over the other" in terms of multiplatform release Example blaster master zero for 3DS and switch or Legend of Zelda Breath of the wild, Wii U or Switch, there has to be enough of a difference (Not speaking about performance, framerate or graphics) 3DS has 2 screens and is still useful over the one screen situation. Once again is up to developers to decide how to make use of games to give it a reason to exist on different systems and not just one. Having a map or quick menu on the touch screen is simple than repeatedly pausing the game to view the map from a menu. To be honest is all dependant on the player and how much they care about the every little bit of detail between the systems or games to decide on what they like.