I think you're on to something Rick. I am building an RV with GRT and have upgraded once along the way. I also am flying an RV with a Dynon. I've dealt with GRT a lot more than Dynon so that may affect the comparison.
Dynon has been very responsive, answering questions and confirming or correcting my understanding of their D-100 unit.
GRT has been adequate at responding to questions. I can count on never getting a phone call returned (days go by w/o a return call and I don't mean during Oshkosh or SnF), which irritates me (note: I have not called within the past 9 months and I know they've added staff). When I do get someone on the phone, I get all the time needed to answer questions completely. They're nice people, too.
From what I can tell of the products, the D-100 is an excellent no-frills EFIS. My expectation is the Horizon HX is going to blow it away in terms of performance and features (and for the cost difference it should).
I've owned a few Garmin handhelds and I been serviced by Garmin. Let me just say, I'm gritting my teeth as I put a Garmin stack in the RV I'm building. If there was a reasonable IFR option, I would have taken it.
So here's my rating.
Functionality
1. GRT
2. Garmin
3. Dynon
Customer service
1. Dynon
2. nobody
3. GRT
75. Garmin