Pretty much opposites of each other. EDR1 is neutral/thin/bright. Depending on how someone wants to classify it, it's somewhere in there. Zircon is bassy and thick, the highs are understated but, surprisingly, don't get lost in all the warmth.
Good sized soundstages on both. Zircon is probably bigger, despite the bass and warmth, but I'd have to A-B them again to be sure.
EDR1 has a preciseness/analyticalness to it, while Zircon is more airy/diffused and has an enveloping aliveness that makes it fun.
I think I've seen some people say the EDR1 has a lot of bass. Either mine is broken, or we have a different definition of a lot.
Some people say the Zircon doesn't have excessive bass. Either mine is broken, or we have a different definition of excessive. The extra bass and warmth is fun on songs that don't inherently have those things, but it's too much for me on bassy songs.