The 7221 uses multiplexing whereas the 6951 uses charlieplexing?
Yes. The 6951 makes it less complex, but the part itself is hard to work with due to its physical size, tightly spaced pins, and only wanting to really drive 3.3V. I think it is intended for small displays.
The 7221 is much friendlier to work with physically.
For example, compare this to the MAX7221 post;
You save 7-8 wires, but effort involved in doing that is high unless you have a PCB readily available to put the max6951 on to, or a DIP adapter like something from Logical Systemshttp://www.logicalsys.com/painfo-vpasp-vb.asp?adapter=pa-qsd3sm18-16
This doesn't provide a pad for the bottom of the max6951, so cooling might have been part of my problem. Soldering it onto the adapter was a challenge.
If you don't have a display yet, I guess you still have options to consider :-)