I thought if you're using MDF and other man made materials, you definitely have to use the M-rated extractor.
Ok here is an excerpt from axminster tools dust extractor guide page:
"L Class (low risk) dust extractors are designed for processes involving mica, china clay, gypsum as well as simple household dust and materials such as soil and lime. The L-class dust extractor is designed for general work. If you are routing/sanding MDF, manmade boards or all solid woods (oak, beech etc), you should use an M-class extractor."
I think L class is for general cleanup like cleaning the floor. I just went ahead and paid the little extra for the CTM. I know I will definitely route and sand hardwoods. Pay the extra now, or buy another extractor later.