I have the Festool CTL 26AC. It shares the same filter system as the CTM 26.
Basically, in terms of the level of extraction, there is no difference between the CTL and CTM extractors. They both have the same motors, same filters, same bags. Both are equally good for wood dust extraction.
The only difference is that the CTM has a sensor & circuitry that provides a warning if the airflow through the inlet falls below a certain level. You only need that if, for some reason, you need to absolutely guarantee that the suction doesn't drop off (due to a full bag, blockage etc) without you knowing about it. Chances are, even without that warning, you'd know if there was a problem!
The CTL is approved for dust category L; the CTM is approved for dust category M. Having the warning sensor is, presumably, a pre-requisite for approval for category M, but since the components are the same the CTL must therefore also be suitable for dust category M, it just can't be approved for it.
That's my understanding of it anyway.