Hard to say. Some people love the 3DS but I sold mine a few months after ALBW released, I thought the system paled in comparison to the DS.
PSP is insanely easy to softmod but the library hasn't aged that well, at least to me.
If you can find a 3DS for a decent price, I say go that route and mod it, specifically for the DS library, it's so effing good. You also gotta take into account the 3DS and New 3DS model (the latter has a 2nd analog nub and is more powerful, standard 3DS won't play all games).
DSi is also insanely easy to soft mod. Those don't go for a ton.
Please keep in mind, unless you get a 3DS XL, a regular 3DS/DSi and PSP/pspGO are going to make your hands numb after 45-60mins. It's really hard to play either for a long period of time if your hands aren't used to it. Those things are frigging tiny. However, a pspGO will allow you to sync a PS3 controller to it, but you need an actual PS3 in order to do so. pspGO will also let you play PSX games in native 240p output to a CRT via component cables. It's a really cool way to revisit classics if you have a CRT w/ component input.