Welcome to the boards! Really good questions and no one answer, at least from me.
I am a piecer, not a quilter -- some people can do marvelous things with a boring top and improve it by quilting. I can take a good top and downgrade to "functional but adequate" by my quilting choices and abilities... Although I strive to do my best and get better, I am more concerned with will it survive the wash rather than will it win an award for quilting.
Every top has a multitude of right/perfect choices. I'm starting to learn that a strong pattern can take less intricate quilting and sometimes a simple serpentine stitch grid is the perfect choice. Likewise, intricate quilting can get lost in a busy print but be perfect in a solid. My last top though, it needs some quilting design in the white/empty spaces of the quilt and wants some sort of all over treatment.
When I stitch in the ditch, I stitch in the ditch and not next to it. Other people feel differently.