Very happy with the TT31. I did think about upgrading to the GTX330, but with the additional cost of the Garmin unit, plus a load of rewiring it ended up being a no-brainer. My engineer slipped the Trig into the KT76 tray without any bother. It works beautifully. To be quite honest, I think the knob is probably a better bet than buttons if its bumpy up there.
The only thing I don't like is the creepy fact that since I had Mode-S, someone in Birmingham is posting all my squawks and altitudes on the Internet (along with everyone else's I hasten to add). Not that I'm spotter bashing. I think its great when these fine fellows come to the airfield and chat about aviation or take nice pics and post them up on the Internet. Collecting Mode-S altitudes however, has gotta be well up near the top of the 'Sad' league table.
MODE-S Reports BHX/EGBB