Well, no-one else has responded, so I will.
I use Omniplan for full-blown project management (which is what I do for a living) - by which I mean large exercises that break down to multiple interlinked activities - anything from 50 to a couple of thousand.
I use Omnifocus for individual activities, grouped, but not dependent, activities and small "projects".
For example, writing a business case is, for me, an Omnifocus small project. It has about a dozen activities (estimate costs and resources, prepare benefits case, outline project strategy etc). For something like this, Omniplan is overkill.
The project for which I wrote the business case would go into Omniplan, in which I have fine control over dependencies between tasks, and I can associate resources and their costs and availability with the work.
My style of project planning doesn't always go down to the finest detail of e very little activity. So if I had a task in Omniplan "Develop training plan", which was mine to perform, I'm likely to leave it at that in Omniplan, and create. Project in Omnifocus to break out the contributing activities, which I tick off as I do them.
So, in sum:
My activities, on a day to day basis - Omnifocus
Big project, multiple resources, critical dependencies - Omniplan
Hope that all makes sense