Well, if you want to know what people think of the various units, then
Harmony Central is your friend.
Here's what they have to say about the
DL-4
Ease of Use 7.8 (281 responses)
Sound Quality 8.3 (279 responses)
Reliability 6.6 (228 responses)
Customer Support 6.3 (90 responses)
Overall Rating 7.3 (274 responses)
And about the
DD-20
Ease of Use 8.3 (94 responses)
Sound Quality 9.1 (92 responses)
Reliability 9.2 (74 responses)
Customer Support 7.2 (18 responses)
Overall Rating 9.3 (89 responses)
As you can see the DL-4 seems to score lower then the DD-20. But then again it has more user reviews, indicating more people use it. Over the past days I spend a lot of time lusting over pictures of pro-guitar players pedal boards and the DL-4 seems to crop up a hell of a lot more then the DD-20.
All I want is a good Edge sound!
There is no easy answer as Edge has such a complicated rig setup that it's nearly impossible to simply approximate it. I used to own a Pod XT, I'm thinking of getting an XT Live for home use. There are an awful lot of user patches out there. I used to try out a lot of them and I found that some were good, a lot were crap. Creating a patch with an XT hooked to a computer is dead easy though. The XT Live has the advantage that it has a lot of effects build into them you don't need to get separately.
Costwise an XT Live runs at around €400 new where I live. You could not get a DL-4, DM-4 and a chorus pedal for that same price. And that's excluding cables and power supplies. So on that basis alone go for the XT Live.
That being said, there is something about separate pedals that you don't have with an multi FX unit. You can turn and twist your pedals on the fly, coming up with unique spur of the moment sounds that is near impossible with an XT Live. Also you can screw open your pedals and mod them to your liking. Although the last thing is a bit hard with Line 6 gear, as its all digital modeling, not analog buckets and bolts.