Wow... you seriously don't know what you are in for.
The comparison is very difficult to make, since you are talking a pure PBX (SX200) to a Hybrid Key System, it is literally night and day. I would seriously suggest getting some training from Mitel, the familiarization and end user training plus the regular technical training... seriously, I don't even know where to begin to give you a comparison and I have been using and installing the 200 for over 20 years, and the Axxess/5000 platform for a good 10 years.