I would ask if these are your only choices?
If so, I have instructed on both and has been said by others who have instructed on both, they both have pros and cons, the Cessna is more cramped and has only one fuel selection (apart from OFF), so does not teach fuel management or the habit of turning on/off the fuel pump (again, not fitted being high wing). Pa38 has better vision out of the cockpit, but poor for teaching trimming as it does not have the trim changes due to the T-tail (good when you have learn't but IMHO, bad on a trainer) both have AWFUL control response. both are forgiving and easy to fly (in spite of what some say) - though this is not necessarily a good thing in a trainer (dH82a MUCH better here - forgiving, but hard to fly well, so made good pilots).
If you really want to learn to FLY rather than just drive round the sky, find a school that teaches on the Robin! Even better, Bulldog or Chippie!!