@rnv I have used both ... currently back in Omni land. My simple summary ... Things is beautiful and Omni is functional ... the basic choice for me was form over function.
Out of the box and easy to get your head around Omni functionality and powerful with scripts and extensions.
Things out of the box not as rich and to me ( a non tech) the things extensions are waaay too much ... though undoubtedly powerful.
One other big kicker. I use calendars a lot and the ability to view your calendar ... with the tasks in the same view - at the right times was invaluable ... not possible with Things .. I asked ... they expect you to live in the Things app and view your calendars there. Like with Omni ... but the views are too limited.
Also @rosemaryorchard published a script that takes that a step further ....