I really like my DD-20. My son has the DL4 so I was able to use his before making my decision. I went with the DD-20 becasue there are more presets and I really like the ability to see, and dial in, the exact tempo of the song using the delay time knob and display. The DL4 has no display. I also did not like the fact that unless you save your current settings to a preset, you lose them completely. The DD-20 has the first preset as a manual setting which allows you to keep those settings even after switching away from them.
I second the motion for the Boss DD-20. A very versatile delay, easy to use, and very quiet. With a Boss FS-5U Momentary switch, it's a breeze to Tap Tempo without holding down the DD-20 2nd pedal for 2 seconds. I always tap tempo the delay time with the FS-5U at the start of each song. That way, I'm always ready to use delay if I want to.
Another vote for the DD-20. I had TWO DL4's die on me seperately...at the gig no less. Before that, the switches had fallen apart on both (yes, I tightened them on a regular basis). It's called a STOMPBOX!! Got the DD-20 a couple of weeks ago while on the road, and even though I'm still tweeking back and forth, it's great, especially being able to see the settings.