GRT and Garmin
I bought a 2007 RV-9A with a GRT Horizon WS, the old AHRS, the old analog magnetometer, and an EIS 4000. IFR gear was a 430W.
I added a Garmin G5 as a backup attitude indicator, and to drive a Garmin autopilot with the 307 controller.
I decided to upgrade the Horizon WS system to a Horizon EX with the new adaptive AHRS and the digital magnetometer. The AHRS was truly a plug and play swapout. The magnetometer needed very minor work to install (I had to create a new d-sub connector that was female and that used fewer wires, basically). The EFIS box itself is the last part of that project and will require a bit of surgery since the mounting screws for the WS and EX are in different places (goodness knows why).
I'll also lose the VOR/ILS indications that I currently get on the WS unless I spring for an additional AIRINC module and do the necessary panel spelunking and wiring. Probably won't do that because I'll still be getting GPS from my 430W indicated on the EX, and because the G5 indicates everything.
Anyway, the upgrade process (combined with my ignorance....) has regularly exposed me to GRT technical support, which I would rate as EXCELLENT. Their manuals, etc., are not quite as good but have shown very marked improvement lately.
Bottom line, I would go with GRT again in a heartbeat.
Garmin... Well, I am a huge fan of the Garmin G5. It's enormous bang for the buck, and the backup capabilities are outstanding, especially with the integrated battery (which I had to make use of one night, long story). Quality of manuals etc. has been very good.
My experience with the autopilot, and Garmin customer support for that hardware, was *considerably* less pleasant. Long story short, I'm convinced that I and others in the experimental world paid Garmin for the privilege of beta testing that system. The best that can be said is that it's working now.
Given that experience, and Garmin's policies with respect to repairing 430Ws, I don't think I would go with Garmin again.
Unless I needed another G5.