PSP/vita tbh
the ds and 3ds are fine handhelds, but the vita and psp completely demolish them where it matters for me: battery life (vita's sleep mode is god-tier), graphics and dear god its library. it's a JRPG/vis novel wet dream, and also a very top-notch portable ps1 if you so choose to use it that way. it's a shame ps2 classics never hit it, because that would make it even better. it's also an indie dream machine. indie devs love it, because vita owners are loyal as fuck, so if you like indies, well. there you go.
the 3ds is still hindered by nintendo's archaic 'downloads locked to console' bullshit, bad graphics, god-awful battery and while the library is varied and vast in comparison to the vita ('specially physical releases), i don't care for 90% of the variation. i hate mario kart portable games, i hate the entire idea of smash on a handheld (or any fighting game, really, tho vita also has a nice list of fighting games if you want them), i don't really care for mario on handhelds, either. 3ds is mostly, for me, for more 'casual' experiences... story of seasons, animal crossing... then there's monster hunter, it's one real redeeming 'gotta have that' for me. and SMTIV being an exclusive. i'm losing a lot of interest in pokemon (still don't have pokemon go "because of server issues" but tbh, i'm really worn out and drained on pokemon) so i don't even know when i'll bother with sun or moon.
as for DS, i wasn't really old enough to build a DS collection, because i hate buying cart-only. same for PSP, but PSP all still have their cases courtesy of gamestop not chucking them in the bin, so it's much, much easier to collect for. local game shop here has tons of great titles for cheap, too, all complete... when i get more cash, i'll be buying out a lot of it. at the time, though, i still looked at nintendo handhelds as "pokemon playing devices"
and 3DS's bc is fucking trash because screen resolution issues and the D-pad sitting so low on the console. people like to cite it, but imo, it's just as pesky as vita's, just in different ways. i know there's a way to manipulate it to the original resolution, but i forget how every time... and i have a DS lite for that anyway.
i know sony has zero plans of another handheld, but i find that a fucking mad shame.