Vita gets a lot of that old-school, downport the console version style of support, if I'm remembering it's line up correctly. (The big games early on were like Little Big Planet, MLB: The Show, GoW, and Uncharted, right?)
3DS gets more unique titles (DKCR3D, Xenoblade 3D, and Hyrule Warriors 3D notwithstanding) because of the hardware.
Anything not getting ported between the two is probably either a) Nintendo/Sony exclusive or b) not worth the effort of porting.