Having both the DS and PSP, I eventually want to get both next-gen handhelds, but am currently waiting to get two of the three next-gen home consoles first. I really want to play Fire Emblem, Pokemon X/Y, Persona 4 Golden, and Soul Sacrifice, so it's been difficult keeping my wallet closed.
Despite the 3DS/2DS being the cheaper option, I have so many Vita games I have free access to due to my PS Plus subscription throughout the past year that it may actually be a better investment in the short-term to get the Vita for me (long-term depends on future titles, whether certain Japanese games get localized, etc.).
As for the question, either Dragon Quest XI (although I'd prefer a home console release, regardless of platform), a The World Ends with You successor, or a 2DS XL (cheaper version of a 3DS XL without the 3D capabilities) would probably be enough to push me over the edge and buy one.