As an instructor who has flown all these aircraft I would make the following observations.
Pa38 - Good for stalling/spinning, good cockpit layout, lousy handling (sensitive in pitch,poor in roll), high tail means there is little trim change with power/flap changes (good on a tourer, not good on a trainer).
if you can land a Tomahawk, you can land anything
- not sure why anyone would think this - very easy aircraft to land and actually one of its good points as it is an easy aircraft to teach people to hold off properly.
Pa28 - Generally OK all round but not actually
good in any area apart from being a very easy stable platform.
Out of the two I would probably pick the Pa28 to teach someone on, but add in at Pa38 trip during stalling, as said though the Pa38 will probably be cheaper.
Ideally I would not teach on either but would rather use a Robin (or indeed almost any other trainer apart from a Cessna
)!