If you're using them as delays, then, either should be good.
But if you want all the special effects and all that, just get an mfx.
No point looking for a delay, cause an mfx has it all.
I use the DD-20, it sounds good alright.
Can't comment on the sound quality as I've never gotten to A/B them. Both have 23s of delay time. The Nova Delay has a better display (choose between BPM and milliseconds) and more presets. DD-20 only has 4 presets but it has a very basic looping function and some weird delay modes like Twist (hold down a footswitch and your delay rapidly increases in speed until it's a oscillating hum).
I think the Nova Delay is damn chio but can't afford one since I already have a DD-20 haha