I think it depends if you're more into 'app' games or 'console games'.
A tablet would be great if you're into application-based games. A tablet can also give other functionalities if you want them (e-reader, portable internet browser, testing device for programming, the list goes on).
The DS line is great if you like portability and also want to play games they've already developed and released. The biggest downside is, with the release of the Switch, less and less games are being developed for the 3DS.
Personally, I have a 1st gen 3DS XL and I enjoy it immensely. I'm still playing games years after they've been released. I'm also very much a 'gaming curator' so I have a very small, but highly played collection of games.
Really, it all comes down to figuring out your portable play style, and how it fits into your current routine. Set out a list of pros and cons for a reliable tablet, a DS, and the Switch, then see what you get. If you find out that the winner happens to be a Switch, you might want to wait out for it. The plus side for doing that too is that you might get a sweet deal when you're ready to buy, such as a bundle pack.