imo, N3DS is only worth it if you really want to play the newer N3DS-only games, if you have games that make use of the C-Stick and you really want to use it, or if you absolutely want to play most of those RetroArch emulators that don't run as well on O3DS.
Don't care for the N3DS exclusives, who needs the C-Stick, and I got an EZ4 for GBA games (and SNES games because the ones I'm interested in have GBA ports.)
I guess there are other systems emulated by RetroArch that work a lot better on N3DS, but I don't think they are all full speed yet (PS1 mainly.) Might become worth it once the emulators get perfected and run full speed, but honestly, used PSPs are reasonably cheap and run the vast majority of PS1 games just fine.
I can see the appeal of wanting to have everything on one console, though. Like when you have to go on a trip somewhere or something for extended periods of time, more convenient to only have to carry one system around.