The M and the MX are very different.
The M is geared more towards TDM - in terms of PICs available and overall performance.
The MX is geared more towards 10Gb Ethernet - in terms MICs available and overall performance.
The major difference is that the MX uses the latest Trio chipset which is driving the unbelievable performance.
The MX-80 applications are geared more towards 1) metro ethernet aggregation 2) carrier ethernet aggregation 3) provider edge and 4) mobile backfaul.
There is a services slot in the rear of the MX-80 - but it's going to be a future release.
The MX family also supports full L2 protocols and IRB.