first time poster, long time lurker.
Thought maybe I could help.
I own both units, and if it's for your studio, or will end up in the loop of your amp, I would say go for the echo pro.
It has all of the features and models of the DL4 plus a few nice extras such as:
It has balanced XLR ins and outs. You can use it mono or stereo with the unbalanced 1/4 inch jacks as well (no TRS though, seperate right/ left ins and outs).
Some other advantages are its ability to sync to MIDI clock from your DAW or another source. (the unit has a MIDI in and an out which can also be used as a thru port if you need to daisy chain)
On top of that, more hands on control, still has the expression pedal input on the back, and more room to store your presets. The A/D converters sound a little cleaner to me on the rack unit vs. the pedal also.
Hope that helps
-Alex