Ive run both. My 1st pre was the AD-20 and then i moved to the UA-5 Pmod (Oade) then i purchased a Wmod ua-5 also.
From my recollection of the AD-20, a few of the downsides were functionallity.
1) It had smaller knobs to control the gain, and no peak level warning.
2)
Only digital out is an Optical outIt has both Digital outs. It does have a COaxial 1/8" out. Dont know if i knew that at the time. i then purchased an M-Audio CO2 converter (which i happen to have for sale here:
http://taperssection.com/index.php/topic,98585.0.html Thanks Jason for correcting me
3) Power supply (runs on 9V so i would always have to change it)
5) 44.1 sample rate maximum
6) 20 bit maximum
7) XLR Inputs only
Some positive things i found about the AD-20
1) Price. I think it cost about $200-250.
2) Size. Good for Stealthing, or smaller footprint.
As for the UA-5's that i owned.
Positive:
1) 96 sample rate (although i never ran it higher than 48.
2) 24 bit.
3) easier knobs to control gain.
4) Peak level LED, although it was still not as precise to monitor exact levels. I believe the flashed at -3
5) Multiple Digital outs (Coaxial and Optical) which were both active, enabling to run 2 digital outs at the same time.
6) Mulitple Inputs: XLR's and RCA's which enabled me to make "On the fly" matrix recordings with SBD and Mics.
7) Phantom Power
Negative:
1) Gain knob for 2nd input is located in a bad spot on the rear of the unit, which makes it difficult, but not impossible to adjust.
2) Price $550. *Not that its expensive, but if you are comparing it to the AD-20, it is almost double. I however thought they are worth every penny.
I liked the UA-5 so much, and thought they were very reasonably priced that i purchased a WMod version, and would decide based on the band, which i would run...the Pmod or Wmod.
I believe the UA-5 is a perfect and logical upgrade from the AD-20.
Hope this helps.