The one advantage to a piston is that it's super simple and a bit less susceptible to debris clogging the orifice. That's about it's only benefit. The disadvantage is that a piston is only "right" for one outdoor temp, and all other temps are some form of compromise which can greatly affect efficiency.
The advantage to TXVs is that they adjust the flow relative to the pressure differential for any outdoor ambient load; a far better system all the way around. They are a tad more susceptible to debris, but this is a function of the cleanliness of the install job. If the install is done well, TXVs last a very long time! Here's why I say that cleanliness is key to reliability ... Metering devices (in this case we're talking TXVs) which are factory installed (such as packaged roof-top units) use the same TXVs which are put into traditional split systems. Because the packaged units are factory sealed and charged, the failure rate of TXVs is very low. However, split systems which require field install (lineset brazing, vacuuming, charging) have those same TXVs fail at a rate about 10x (TEN TIMES) more often! So it's not the issue of TXVs being unreliable; it's the topic of sloppy install work in the field.
I recommend you get the TXV, and then make sure you talk with the installer about the necessity of a super clean job.
