Update: 2013.06.28
My life has sub-projects. Nirvana and Things don’t. It has become a problem. I’m not interested in simulating sub-projects with tags. (It gets awkward fast.)
OmniFocus has sub-projects, but it doesn’t have tags. That’s a problem because I want to tag some of my tasks with my boss’s name or his boss’s name or HIS boss’s name. I need to be able to find all the thing’s that an executive has put on my plate, for when I talk with him. This is not a single-context solution, because I’m not going to work on the item in the exec context – I’m going to work on it in some other context and I have to report on it in the exec context.
 
I have to be able to have either tags or multiple contexts. OmniFocus can’t do that (but I have a hack for that below). I have to be able to have sub-projects. Things and Nirvana can’t do that. MyLifeOrganized does multiple contexts (which works like tags) and subprojects.
 
To hack OmniFocus to simulate tags, enter a unique string, either in the task text or in the task note. In my case, I could use “[tom]” or “[richard]” or “[bill]” (or I could use “[executive]”). I can use OmniFocus search to search for these, and I can create OmniFocus perspectives to display all exec tasks.
 
Or I could use MyLifeOrganized (but it doesn’t do AppleScript). And it is ugly (IMHO). And it doesn’t work real elegantly with a Mac keyboard because it likes Insert.

I’ve been using Nirvana (2 - beta). It works well for me with some exceptions:

• I'd like to see Areas in the sidebar, similar to Things.
• I'd like to have Folders, similar to OmniFocus. (This would make Areas irrelevant.)
• I'd like to have an AppleScript API. (I've cobbled together a quick-add AppleScript for Nirvana.)
• Uh... When are they going to start charging for it. Free isn't a long term business model. </ul> Turning things around, Cultured Code's Things looks good except:
  • I'd like to see tag clouds or to make the tags window capable of filtering based on selected tags similar to Nirvana. (Global search-by-tag, by picking the tag, not entering it in the search window.)
  • I'd like to have Folders, similar to OmniFocus.
  • The length of time they took to release cloud sync leaves me concerned for their long-term viability. </ul> OmniFocus has these shortcomings:
    • For Mac+iPhone+iPad it is mighty expensive.
    • Some people say it is too complex. I need to evaluate that for myself. </ul> Some givens:
      • I'm going to use a GUI (i.e. not todo.txt)
      • I'm going to use it on my Mac and my iPhone
      • I have to be able to get my data out of it and into another tool at some point in the future. </ul>