True, the TT31 is a direct replacement for most KT76 types. However, if you want to connect the ADS-B position functions for the extended squitter, you'll need to have some rewiring done to feed the position data from the GPS to the transponder.
True, Mode S with ADS-B makes you visible on sites like flight radar 24 or others. So what, everyone else is. Personally, I prefer having technology on my aircraft which will prevent additional expense in the not too far future, when the powers that be will decide that ADS-B becomes compulsory one day, which is really only a question of time. Plus, I can't really figure why to buy an inferior product which costs more.
I'd do the same thing again and I have given advice to people who wanted to upgrade their Mode C transponders to do the same. So far, Trigg has come out very well indeed.