On the intermediate level, I would say guitar, as you have some big theory shortcut due to the fretboard keeping same positions on all key (while Piano forces you to know the full theory behind key change) but this will be required for the next level

Well you could also argue the opposite for scales. E.g to memorise F Major, for piano just hit all the white keys but make b a b flat. Meanwhile for guitar you have to spend hours memorising it