After trying lots of pedals over the years at 18v, I am definitely a 9v man. Sounds warmer and less bright. Just my preference. Plus it is way too much of a hassle with my pedalboard because I would have to pull another pedal off my board to run one pedal at 18v.
I play run a Fulltone Fulldrive Mosfet at 18v. I think it's much better. It's more open. Maybe even has a bit less gain, but it's just more musical sounding to me. Of course, not all pedals benefit from 18v. Also, the LED light is crazy bright with 18v!
18V is really best for a couple specific conditions:
1. More output if you want to drive the front-end harder
2. More headroom in the high voltage loop for clean boost
The signal coming out of the guitar pickups is around 50mV. There is really no need to run 18V for that unless you are using a TON of gain before the preamp. A gain of 180 (45dB) gets you from 50mV to 9V. 18V gets you another 6dB before you saturate the pedal.
The one place you could argue that 18V matters at the guitar output is for transients, but I think that 1V would be an awful big transient from a guitar pickup. And it's going to limit in the preamp anyway.
As a matter of fact, 18V sounds different than 9V. 9V is smoother and darker. But with the ED, I feel the Xotic on 18V complements very well, at least at low volumes, which is the only way I can play these days (I am looking for a new gig though).