Evernote and OneNote are direct Competitors - Long Time Competitors.
It honestly boils down to a couple of big differences:
1) Evernote is cross platform. There is an Evernote client for almost every mobile platform as well as many desktop platforms and web.
2) One note costs money up fron, evernote is free but has an optional premium plan.
My personal preference is OneNote's Desktop app, but Evernote's cloud based service. I like that I can email a note to my evernote account and it shows up in my list. Evernote has lots of integration points with other software packages and has some nice cloud features like OCR.
Onenote has a KILLER Windows program. Hands Down one of the best on the market. But you are stuck in MS' world completely. the Web Client is nice but a bit awkward compared to Evernote's Web stuff.
I use both. If I had to only use one, I might go with Evernote because of its broad support.
But short answer is that its all about the details really. Each has its advantages and disadvantages.
The Evernote Client for WP7 SMOKES the One Note Client, though. (IMHO)