I'd go for the ML-2 if you're looking for Mesa sound and the MD-2 if you're looking for a very overdriven Marshall. It all comes down to personal taste and personally I would choose the ML-2.
Okej most people prefer ML-2. Whats bad and good when we talk abaut a MD-2 then? Does it fit better to british steel music and heavy metal rather hardcore?
Boss is sometimes known to have sort of misleading pedals. the MD-2 isnt exactly the distortion monster its made out to be in my opinion. i certainly wouldnt use it alone for metal. i use it set around half with gain 2 completely off driven by a SD-1 followed by a EH-2 for my metal rhythm sound. but the MD-2 used alone is more in the DS-2 ballpark i would say