Can you give me more information? Which markets are each targeted for?
Thanks
That's a tough question to answer but I'll give it a shot. Keep in mind this is coming from a guy who flies a 30+ year old 172 with a G400 and steam gauges...so take it for what it's worth. I've been reading/studying/dreaming about these avionics for a few years though so I feel like I can offer some insight. The people who are actually flying behind these systems I'm sure will chime in with insight into how they use their systems and why they chose them.
Dynon - focused on the low cost end of the market. They provide a lot of functionality at an impressive price. From what I've gathered from the opinions expressed here on VAF (I'm generalizing) is that it is a system more suited to VFR than IFR. This is mainly due the lack of the ability to integrate with other avionics. For instance, Dynon can not send GPS steering commands to another company auto pilot. Also, in the past (this has been updated in the latest software release) the attitude display was dependent upon having pitot data. If you lost the pitot tube for some reason (icing, debris, etc) you would lose accurate attitude information. The system now uses GPS ground speed data as a backup so the attitude indicator doesn't go TU if you lose the pitot data.
The Skyview system is not reliant on pitot data for attitude solution and if the pitot is lost then reverts to GPS ground speed and the attitude information is still good. They are working on developing GPS steering commands their own autopilot and should be available in a future version of Skyview.
Grand Rapids - Based upon the opinions I've read here this seems to be the preferred avionics for IFR flight. Search for "GRT and Ironflight" in the archives and you'll find some excellent write ups from Paul Dye (aka Ironflight) on his experience with his GRT system.
The GRT system is about double the price (maybe a bit more) than a Dynon. It's advantages are it's ability to integrate with a lot of different boxes from a lot of manufactures. It can take the flight plan and GPS info from a 430 (or even the 39X/69X handhelds) and feed GPS steering commands to your autopilot. It takes traffic information from a Xanon or GTX transponder and can overlay the traffic targets on the moving map. With an XM weather receiver you can dispaly weather info on the moving map.
The attitude information is not dependent on the pitot data for the solution which makes people feel more comfortable about flying it IFR.
Garmin - The highest priced route but you get the comfort of buying from a "major" manufacturer with certified experience. Keep in mind none of these systems from Garmin are certified though.
The G3X stuff - the low end of Garmin's offerings. The price seems to be equivalent (maybe a bit more) than a full GRT system. However this system doesn't have the equivalent level of functionality of a GRT system. I don't think (someone please correct me if I'm wrong) that a G3X can provide GPS steering commands to an autopilot. Obviously it does integrate very well with other Garmin products.
The G600 - I don't really see how the G600 is different from the G3X stuff. It appears to have about the same functionality but the price is much higher, about double the G3X. Someone who knows this better can elaborate.
The G900 - in my opinion this is the Cadillac of avionics. Nice big screens and a truly integrated system. You get the radios, transponder and GPS all in one package and it works very well together. The G900 will send the steering commands to an external autopilot as well. However this is very expensive, about $60K from what I can tell (again someone please correct me if I'm wrong). This would be an excellent system for IFR work with as close to certified hardware as you can get (it's basically the same system just without the paperwork) and has been proven as a good IFR platform on literally 1,000's of airplanes.