I was a huge OneNote fan since I started using my Lumia 800 back in 2011, but I switched to Evernote a few months ago. The key advantage I see in Evernote is not just the ubiquity across platforms as well as services, but the speed and frequency with which the company iterates its products.
Evernote for WP may have started out a poor, bare-bones cousin of its iOS and Android siblings but they have actually put in effort to improve it to the point where the Evernote app for Windows Phone today is actually more powerful than OneNote Mobile. Meanwhile, OneNote Mobile has had practically no improvement since debuting on WP7 and WP8 respectively (WP8 users correct me if I'm wrong, I don't know if Microsoft has ever updated/improved the OneNote app for WP8 since launch).
Evernote has even updated and improved its Windows 8 touch app really quickly over the course of a few months. OneNote for Windows 8 is really good too, but like the mobile version, it seems to be at a standstill. I also have an Android tablet; don't even get me started comparing Evernote and OneNote for Android...