I have both. They sound quite different.
ON-1 has a really pumped up bass. It can often sound unbalanced. But if you're a basshead or listen to music that benefits from ample bass, these are up your alley. The sound is powerful and weighty. Very meaty sound that can verge on sounding a bit unrefined, comparatively. Very bold and slightly brusque, but a good sounding can. Comfort is so-so; memory foam pads, but on-ear with a decent amount of clamp. Isolation from outside noise is good. A novel and unique headphone with a powerful, bold sound.
The ESW9, by comparison is more balanced, smoother, and more refined. Not as exciting, but makes the music sound beautiful. Lush and musical. On some systems it can sound too warm, but In the right system it's a musical and beautiful sounding headphone. Comfort is great; lamb skin covered on-ear pads, comfortable headband, and very moderate clamping force. Isolation from outside noise is not the best, however. An elegant looking, feeling, and sounding headphone.
When listening at home, I prefer the ESW9. But when I'm out at a cafe, I often prefer the ON-1 for the better isolation and bolder and livelier (if rougher) sound.