I have asc tubes and GIK. If you go with tubes then nothing smaller than 16” and really 20”. The GIK stuff works. I think the soffit is 18”. If you install this around the perimeter of the ceiling tge results would be profound. It has great absoption down to 80 Hz and still good absorption at 60Hz
If you are handy then building a soffit trap is easy and would save quite a bit
I recently tried ASC tube traps (big diameter base and smaller diameter top) and was shocked at how powerful they were in a small room. I almost returned them because they were too powerful and not for the better in my room that was fairly well dialed in.
However, I split them apart into 4 pieces (which is a nice option) and experimented with placement/rotation. Once I found the right place to put them, I'll never have a room without them. They are really great at what they do and can enhance a room more than almost any other equipment purchase except speakers.
I don't have experience with that specific GIK product, but I've tried tons of other things. I'm sure the GIK is nice. It's just hard to imagine them being better or more versatile than the ASC.