I've not got a wealth of experience to base this on, having only just used my AD-20 for the first time, but I have to say that I'm very impressed with the improvement over the Microtrack's own ADC.
I find that recordings made with the MT ADC seem can have a brittle, characterless and rather thin sound lacking in warmth. I would describe the AD-20 on the other hand as being warmer and more musical.
Obviously, this isn't a properly objective opinion, but no-one seemed to have an answer for you so I thought I'd chip in... I have heard at least one person here say that the AD-20 doesn't represent a worthwhile upgrade to the MT's native ADC, but going off my own (one) experience I think I'd have to differ.
I'd be very interested in hearing/doing a proper AD-20 vs. Microtrack comparision, just to make sure that it isn't my imagination trying to justify the purchase