I've never run an ADMIC-20
but I can give you some of the pros and cons of an AD-20
cons:
No level meters
No Phantom power
only 44.1 kHz
Knobs for adjusting levels are really small
minimum gain is +16 db (at really lould shows you have to set levels all the way down and hope for the best, personally I have never had clips that affected playback)
pros:
not very expensive
small in size (croachable)
pre and A/D in one box
both coax and optical outs (great for JB3 users)
built like a tank it won't break if you drop it
easy to power on either 9v battery or will run a very long time on a 9.6v RC battery pack
All and all, I was very happy with my AD-20. I made many great tapes with it. I just got a Warm Mod UA-5 (which I haven't gotten to try out yet) and the reason for getting it wasn't because I didn't like the AD-20, but I wanted to try out the "warm" sound and want the abality to record at different sampling and bit rates.
I think if you are happy with the sound your microphones give you going "Line IN" you will be happy with an AD-20, because the mic pre in the AD-20 dosen't do all that much to the sound. I've heard some people call AD-20s the "thinbox" (instead of the in-box) probably because they were using "thin sounding" microphones
An AD-20 is not going to improve the sound of your microphones, but is a good inexpensive way to add gain and convert analog to digital, without any "extras"