IMO, a DD-20 is almost unbeatable as a general, flexible, do-it-all delay. The Nova Delay is a very similar unit, and of similar high quality. Both units do excellent hi-fi digital, analog and lo-fi tape delay-type sounds, as well as modulated delays and other funky things.
The benefit of the Boss is that you can set it up to change delay settings with just one stomp. With the Nova, it is a bit more fiddly, and requires at least two stomps.
The benefit of the Nova is that it has more programmable delay settings. With the Boss, you can have 4 saved settings plus a "manual" setting. this really gives you five delays to use. The Nova has 9 (I think), but please correct me if I'm wrong.
Personally, I chose the DD-20 for ease of use in a live setting, and quick memory changing. You can add a cheap tap tempo switch unit, and connect it through the external switch jack, giving you one dedicated footswitch for TT, one dedicated footpedal for delay ON/OFF and one dedicated footpedal to shuttle between the manual and chosen memory settings. Hitting both footpedals together scrolls through the 4 memory settings you will have programmed. THe Nova can't touch that level of live user-friendliness. Oh, it also does both -10 and +4dB, so is good for most loops, as well as in front of the amp.