Hi Jordintin,
Also, one has the word "Cloud" it in. jk. All of the links that Tim sent are spot on in terms of the scalability. One thing that is important to my clients is that the 7000s have a single power supply while the 7200 series have a dual power supply option.
The other thing is that the 7200 series can be a master and redundant master for a set of 7000 series locals. The 7000 series "should not" be a master and redundant master for the 7200 series; it's not a supported configuration.
Hope it helps!
-Mike