After having used Things 3 for a year, I've switched to OmniFocus. I've used previous versions of both apps for many years.
Things 3 fails in key areas:
1. No way to easily see which tasks are due and overdue. Things 2 had a switch for that, but 3 doesn't. Major omission.
2. Cultured Code never listens to user feedback, let alone entice communication with their users. OmniGroup on the other hand offers EXCELLENT, attentive support AND listens to their users/customers.
3. Things has no way to save searches. This is particularly problematic since it doesn't has tabs either. Whenever you switch view from f.e. a tag-filtered Today view, to some project, and back again: your Today view is no longer filtered by that/those tag(s): it's back to its default view of everything. OmniFocus on the other hand, has Perspectives, which is like "saved searches on steroids". Immensely useful! One click, and you see your tasks from the exact perspective – filtered exactly like you've configured it.
4. Things 3's sections/headers were a nice, welcome addition to Things 2. But compared to OmniFocus' true, fantastic outlining capabilities: their usefulness is very very small. You can't collapse them, to create a less cluttered view. You can't search for them. They don't do anything else than visually divide your task list into sections.
5. OmniFocus has attachments. Even audio files. Things 3 has none of that.
6. Things 3 dumbed-down their "start-date" feature. And it's not as powerful for postponing projects, as OmniFocus' "defer-dates" feature is.
7. "next actions" in Things 3 is as bad an implementation as it was in Things 2. In OmniFocus, you have true dependencies. Projects can be set as "sequential" – and you'll only see the next action in your action lists. This is HUGE! If you have complete a given client project before you can invoice it, there's no reason that the "send invoice for the project" shows up in your list of admin-work to do. In Things it will. Things is "dumb" that way.
Overall, in Things I end up with way too many tasks in the Today view, because everything I don't wanna forget ends up going there. But the result is a constant list of 50-100 tasks in "Today".
OmniFocus lets me prioritize and FILTER much better – and more consistently.
And OmniGroup is really, IMO, such a cool company worth supporting. For us long-time Mac users, they're one of those developers who has stuck with the Mac, listens to their customers, and develop unique applications that you just KNOW WORKS.