To maintain a task list in Evernote is fine and not especially difficult in terms of keeping the data in the app. For me, it gets dicey when I need to access or manipulate the data, particularly on a mobile device. If I keep my tasks in a separate notebook and tagged them all such that I could call them up quickly with a saved search, I’m still looking at what I would consider a prohibitive number of taps + waiting time.
OmniFocus, on the other hand, is an application whose sole purpose is task management, so it’s built with the understanding that the user who opens it is, in all likelihood, interested in creating or performing a task. It provides lots and lots of ways to slice and dice my task data (including grouping tasks based on my current location) and it’s very, very fast. Depending on how you structure you task list in Evernote, it’d also probably be a bit difficult to get a bird’s-eye view of your “plate”. That coupled with a few tactical-level things like dependent tasks, date-based availability and others are what make OmniFocus a clear winner for me.
(A) OmniFocus is awesome.
(B) That bit about mobile sync is why I don’t use Evernote.