I wasn't impressed with the RE-20 at all and I really wantled to like it when it was first introduced.
The DD-20 wins for me.
for a wide range of basic delay tones, i liked the dd-20. for a wide range of "specialty" delay tones (that cop not just any tape delay, but specifically the RE-201), the re-20 was pretty good.
In the end however, i sold both... and i really wanted to like the re-20, with its ability to use a speed pedal to control the feedback, for ambient stuff. the problem was the feedback / oscillation sounded fake to me, at least compared to my ad-900 that i've had for the past 3 years.