My first reccomendation is always the Ikon2. This thing just works, has bailout mode and self level if you are in to that sort of thing. And, it comes with it stock, nothing else to pay. This will make sense in a minute lol.
The AR7200BX, AR7210BX and all of the other AR7xxxBX gyros are basically an AR7000 receiver built right into a BeastX gyro. These are nice if you are a little tight on space and have no satellites (which I highly suggest). These gyros will accept a single satellite and then use their onboard antennas as the other receiver for diversity. These are a little harder in some aspects to mount properly to get true diversity.
Next is the MicroBeast. This one I would not put on a 550 mainly because it is only a single satellite if you are using the adapter to use the satellite which in some cases you have to purchase separately. My microbeast came with one, and I have the MB clone that I had to order the adapter from china (clone gyro, I was not going to put an adapter cable on it that cost as much as the gyro lol. All it is is a 5v to a 3.3v stepdown so you don't blow up your satellite by over volting it).
Here is the kicker with the AR's and the MB's though (actually I think it is only on the AR's, not 100% sure, someone correct me if I'm wrong here). If you want bailout like comes on the 230s and other helicopters, then you must not only purchase the gyro, but you MUST spend an additional $100 for SAFE, which is Horizons version of bailout and self leveling. This is why I highly suggest the Ikon2. The full version.
So about the ikons. If you have a good BEC in your ESC, then you can just get the standard version of the Ikon2, but definitely not the mini. If you would like the ability to setup over Bluetooth, you can get that version. However, if you are looking into running a separate receiver battery, then the Ikon2 HD is what you want. It comes with the wires and the plug to connect a 2s lipo directly to the gyro and the gyro takes care of the power to the servos. If you do it this way, remove the red wire from the ESC so that you don't have conflicting voltages or even worse, reverse voltage into either the ESC or the battery. There should be diode protections to prevent this, but it blows up the diode.
Lots of info, let me know if you have any questions or if that was a little too confusing.