In answer to your direct question I have not tried both.
However, I do have both on my ST. The Autocom handles the audio interface between the rider and passenger and the rest of the world.
The J&M CB 2003 handles just that CB radio and weather channel.
I have a Garmin 2820, somewhat similar to your 550 in capabilibities which handles the navigation, MP3s, XM Radio, and boothtooh interface if I want to do that.
I chose this combination because the J&M CB was the cleanest looking device. It does come with a hefty price tag but it functions as designed and is easy to use while on the bike.
The Autocom was selected primarly because of all the good press they get here and in other forums. It has proved to be a good value. I have the pro rider, I especially like the background noise sensor and the way it handles the audio volume, the higher the noise level the higher the volume.
The 2820 has a speed sensitive volume system but I find that it's operation is less than adequate.
I like the way the Autocom interfaces with the rest of the equipment, the small size of the microphones and their sensitivity. I don't like the in the helmet speakers all that much and just got a set of in the ear speakers (not exactly the right terminology but close) from Arizona Al. They are really a great investment.
Cost is an issue in all these systems.
The best you can do is ask a lot of questions and see what fits into your budget. I probably went over budget on my set up but I am very pleased with how it all goes together.
All I have to do is to turn on the GPS, fire up the bike, plug in the helmet cord and I am off. A simple turn of the switch on the CB gets me on the air and, if I want, I just go to the settings function on the 2820 and connect a bluetooh device. I rarely fool around with the volume controls.
The only real issue is connecting the J&M to the Autocom. There is no interface cable and you have to "roll your own". But I have a schematic that shows how mine is interfaced if you wish.
jim