from what ive done the rc20xl is good as long as your playing solo, in a band context (without a click track) it will inevitably lose time at some point, i even use mine as a sampler (done original voice intro stuff, the clocks from pink floyd's time, a oscillated note for an intro, and this spacey groove thing). i havent messed with the looper functionality of my dl4 much because i dont need it, i had the rc20xl first and its my dedicated looper (it might have a noise problem, i havent tried tracking it down but im noticing a bit of a hum somewhere in my chain)
post what your needs for each pedal are if you dont mind, i wont recommend the dl4 for looping b/c of its lack of features but for delay its great (check my self oscillation post in the... self oscillation thread) and mine has been (thus far) durable - i think its the old ones you have to worry about
hope this helps