I can't speak much for the G3X system, but I have a lot of experience from Dynon. The installation of the Dynon, especially if you couple it with the VP-X, is definitely a DIY system. If you are planning on flying VFR, then your panel can be completely (or almost completely) Dynon, with their COM radio, intercom, transponder, GPS, ADS-B, Engine Monitor, Auto Pilot, etc. The graphics and the functionality of the system is fantastic, although I think this could probably be said of all 4 of the main players (AFS, Dynon, G3X and GRT). Their auto pilot now has advanced features like glideslope coupling, etc, but this can also be said for all of the 4. I think I would recommend getting a flight in a plane with both options and deciding for yourself which you like the best. I personally like the more eye-oriented screen of the dynon, but if you split the screen to PFD and Moving Map, then it ends up much like 2 G3X screens, except with no space between them. I find that when I am coming into the vicinity of the airport, I like to go into full screen PFD, which gives you a full 10" of synthetic vision, which shows the runway, traffic, obstacles, etc. You really need to price out both options with the full panel included, then list the features you prefer from each system, and make your decision based on that.
One thing I will say, is that you can do a fair bit of wiring in your airframe, especially if you go with the VP-X, without deciding on your system. I have heard so many comments from people who decided, bought, and weren't even flying by the time the vendor discontinued their system. I have wired a complete airframe (conduits already installed) in less than a week, including installing the instrument panel, after the plane was painted, so you don't need to make your decision yet. Engine options won't change in the next year or 3, but avionics likely will, so I recommend you make your decision when everything else is done, especially if you are going to build it yourself.
You don't need to decide on a pitot yet, but you do want to install a pitot mount. Unless something has changed recently, most of the major pitot tubes will fit in the same mount/mast. If Dynon is one of your options, then run a 2nd pitot line out for your AOA. The weight is negligible, so it won't hurt to run it.
Many people recommend that you get your autopilot servos ahead of time and install them before you close up your wings. I completely disagree. With your wings on a stand, I can install an auto pilot servo in a finished wing in 15 minutes. That way you are not cornered into a specific system too early in your build. Installing a pitch servo bracket in the tail wouldn't hurt, but in my experience most servos mount the same way. People with experience with the G3 may correct me on this. I know the Dynon and TruTrak, and thus the AFS also, servos fit the same mounting plate.
I know it's exciting to think about the avionics you will be flying behind, but it's too early for you to make a decision. Find an opportunity to fly behind both, or all 4, for that matter, and let your decision start to develop from there.
