That's the essential trade off between these two pedals, the low end that's coming through (or cut) and how it affects the headroom. The OCD has a bit more low cut, which translates to a "tighter low end" or "more clarity". The UD has less low cut, which translates to "wooly/fuzzy/more saturated" and they react differently between the gain and tone at different settings. That's why I still have both and still like both for different reasons. As far as quality goes, in my experience it relates more to reliability and longevity than tone. I've done blind tests on hand built pedals with the exact same circuit and different components (vintage NOS "mojo" type parts, high end new parts and off the shelf Radio Shack parts) and could hear no discernible difference in tone in a real world setting. Those were all handpicked for specific tolerances/values though, to be as fair a comparison as possible. It's certainly true that it can be a bit of a crapshoot with low end pedals, because of those looser tolerances, one pedal could sound a little different than another. That's true of a lot of pedals though, especially the vintage ones people hype a lot.
