I wrote an article on TRIG and Filser offerings some years back in
Pilot back in 2004 (when Mode S became a topic) as alternatives to Garmin and Bendix.
At the time they were good budget options in lieu of any other LAST (Light Aircraft SSR Transponder) unit, but in the mean time Garmin (more so) and Bendix have reduced their prices, so that you are only saving ££s (if that) rather than £££s.
Coupled with some of the other nice features you get when you add 430/530 etc; we ended up installing the Garmin GTX330 in 3 of the fixed wing fleet we use (because we use them for IFR and VFR; they already had 430s) and the price difference wasn't that great by the time you installed it - so even less of an issue if you fly choppers
Our avionics chap told us what and where to buy. We bought our units from Bournemouth for about £1800-2000 (2008 and 2009) and it cost another £5-700 for installation/paperwork.
Look, feel + usability of the Garmin is excellent. Touch button is excellent for one handed operation as its fast and means you don't have to watch the dial - just punch the 4 digit squawk and the 4th digit entered activates the squawk. VFR button returns you to 7000.
The Garmin 330 is also ADS-B ready (no upgrade path for 328 to ADS-B)...and that is the next thing that will come along (already in the pipeline - approx. 5 years away). The 328 was designed for the European market and is supposed to be a straightforward swap-out, but in practice (correct me if wrong) I seem to remember the rack and harness still need to be installed.
Bendix KT73 is other real option as it can use the same rack as the KT76C (which FEDA has), but is more expensive (as you know), has 'twisty knob action', is no cheaper to install and if you want it ADS-B ready needs the KT76 rack replaced with a KT73 one (which has a 2nd auxilliary connector for these features).
After that you'll want a new 'touch' comm panel like 340 or even PMA units
so that passengers can enjoy music from the ipod...
John R81, I sent you a PM.