Both are great cheats and best thing to do is try both to see if it meets your needs. In regards to safest cheat I would say synthetic skill as its been a while since the last detection and proofcore has just come back after there last detection and months of testing.
Every single day there's another stupid post like this asking the same exact question as if there's some quantifiable scale of "safety" or even which is better. Every provider has its ups and downs and they virtually all have detection history. Close your eyes and pick one then have fun. Inevitably when you get banned go play on a new account and keep having fun.
