Transponder TT21 versus TT22 - Considerations

By all means get clarification on the point he was making, which to put it clearly is that Trig TT-21 is not TSO-C166b certified, whereas the Trig TT-31 and TT-22 are. Trig are usually remarkably responsive.

However a really hard/most expensive issue you face if intending to meet the 2020 carriage specifications (start at AC 20-165A) has nothing to do with any of the questions asked. It's meeting the requirements for a IFR/WAAS TSO-C145c (or similar) GPS source and finding somebody who can work with a FSDO on field approval (which the FAA would ideally like based on a prior STC, which there were none for gliders... but hopefully you start with the same GPS source and 1090ES data-out capable transponder installed via prior STC in some GA aircraft and check in with the FSDO folks first). That TSO-C145c GPS receiver will currently cost you more than a new Trig TT22.

If your glider is experimental then it may be all moot, if you just want to say "play" with ADS-B data-out then you can likely install a non-compliant system with a COTS GPS. You might get a friendly letter from the FAA pointing out you are flying around with a non-complaint system. And whether you have a fully TSO-166b complaint transponder may not be relevant. Lots of things to discuss there with any vendor before trying to do that.

Now if you ask any avionics manufacturer what power-transponder you should install and state you fly above 15,000' expect to be pointed at regulations that state you are required to use a 250W transponder, especially when its clear you intend to copy the world on the answers... even if in practice it mike make no difference.

If you fly in busy airspace, with airliners and fast jets etc. Please install a transponder. Whether it is a Trig T-21 or TT-22 has really no difference for the usefulness of that transponder. If you intend in future to go though all the money, complexity and hassle (uh why?) of installing ADS-B data-out as well then the question about TT-22 TSO-C166b certification may become relevant.

If "cost is a consideration" then that question likely answers itself -- save your money and forget doing ADS-B anything. And my question for folks who worry too much about ADS-B anything starts with why exactly do you want it/what extra service/capability do you expect to receive from having ADS-B data-out or in....