How detailed do you want me to get? I've hacked both of them so I can give you as much info as you want. From a broad perspective, Vita has KASLR/ASLR, stack canaries, trustzone, just to name a few standard things that 3ds doesn't have. Overall the 3ds is pretty much "hacked" (only a couple of components are still secured including bootrom and aes engine). Vita is not even hacked at a kernel level yet.