In the end, I think it is mostly a personal choice about features and ease of use, and I think the only choice is between B&G and RM. Garmin is too new to the game. Go to a boat show or store and use the devices. Check out Panbo's site. Talk to fellow sailors. It's a long winter.
Furuno is another major brand.
Your CP will receive heading data over that connection (for radar overlay) and may control the AP. (Emphasis on the word “may”). Navico (B&G, Simrad, Lowrance) does not play well with Raymarine.
Controlling an AP through a chartplotter requires the AP and plotter to be the same brand (and usually the same vintage) for everyone. It isn't only a Navico thing. Same with radar.
Radar was mentioned earlier. If radar is the most important component, pick the radar unit that works best for you, that will determine which product line you go with. Radar protocols remain proprietary.
This is an important point. However, choosing the radar only determines which chartplotter one goes with. The rest of the instrumentation can be chosen for their individual attributes and mixed as direct plug and play. Except for Raymarine, which will require some converters to use with other instruments.
We have also just finished an electronics overhaul. Since radar is more important to us than the chartplotter, we chose Furuno for this part of the system, and the Furuno chartplotter had to come along. I've used almost all of the radar brands, and radar is one area Furuno stands apart from the others.
After having settled that part of the system, we then chose a Simrad autopilot and controller (driving our old Raymarine hydraulic pump), Simrad compass, Maretron GPS, B&G Triton MFD instruments, Vesper AIS, Airmar speed and depth transducers, and LC Captuers wind transducer.
Like mentioned earlier, Simrad, B&G, and Lowrance are mostly the same stuff, but either limited/differ in certain functionalities, or limited/differ in which peripherals are available in the line. However, their autopilots are exactly the same, and the "differences" only show up through software when one chooses the type of boat they are used on. For example, if sailboat is chosen, then sailing-specific features are activated, and if powerboat, then things like preset steering patterns show up. The reason we chose Simrad here is because Simrad offers a separate full-function control head, while B&G offers a limited control pad that works through a Triton instrument.
Also like mentioned earlier, if your AP drive unit is in good shape, there is no reason to get a new one, nor any reason to make it the same brand. Keeping the previous one in place saves money and time.
Our previous 2015 Garmin 7212 12" chartplotter and 24HD radar is currently for sale if anyone is interested. They work perfectly, and the only reason they were removed is because we were replacing the very old Raymarine stuff and decided to make a clean go of everything. It comes with all cables, GPS transducer, manuals, etc.
Regarding the Garmin CP in terms of some of what has been said above, it is a touch screen only and we never found this to be a limitation or issue - always worked well even with wet fingers. I also found the user interface to be very good and intuitive. I'm hoping the touchscreen usability is not Garmin-only, because we replaced it with a Furuno touch-only.
Mark