Thanks for all the advice on Pocket Informant. Here's a bit of feedback on my experience.

After playing around with hierarchical tasks for a few months, I've got stuck on two limitations:

ADD PARENT TASK
- It's difficult to add a 'parent' task. So, if you create a next action, and then later decide to add a parent task (e.g. a 'project'), you can't just select 'add parent' as you can for 'add child'. The way I got around this was to duplicate the next action, make one a child of the other, and then edit the parent into a project. As it sounds, this was quite a cumbersome method.

TIME CONSUMING
- In addition, I found that I was spending a lot more time than with previous methods on managing next actions and projects. In particular, locating the two tasks that I wanted to link, and having them on the screen at the same time was quite challenge. PI makes it very easy to find one task, but there's no method that I could find for running 2 searches at once.

For example, if I had a project, say, 'Drink cup of tea' in my 'project' category, and a next action 'Put kettle on' in my '@Home' category, I could tell PI to show me all my Projects and @Home actions and I would get over 100 results. Then, if i filter on 'kettle' I don't get the project. Perhaps PI lets you filter on more than one word (e.g. 'kettle' OR 'tea'), but that still requires knowing the title of each. In short, this method is not as simple as PigPog in terms of creating an easy connection between project and Next Action.

On top of these two points, I then found that when I'd created a lot of hierarchical task connections, PI kept crashing. I removed and reinstalled PI, to find that the hierarchical task relationship were not saved in settings! (I should have backed these up separately).

I would happily have put my data back, and investigated the crashing problem with the developers, but combined with the above mentioned challenges, I've decide to switch back to the pigpog method.

Incidentally, I've found that PI makes the pigpog method very easy, as you can edit the title of a task, without even opening the task. Nice!