Quick answers: piston is a fixed oriface which is perfectly optimized at only one set of operating conditions. TXV is a modulating valve that varies the expansion flow of the refrigerant by a balance of liquid line pressure and vapor line temperature. TXV provides a better metering of refrigerant as conditions (temperatures and heat load) vary.
TXV: more complex, can fail due to a number of reasons, better load matching, thus higher potential SEER
Piston: Simple, cheap, virtually no failures other than clogging, less than optimal mereting in varying conditions.