I would do both.....that would draw the eye from the brown corner out to the edge of the border corner. Hope I clear on this, in other words do one white small square and then diangonally another. The one white small square looks a little lost with the beautiful brown fabric around it. Since you prefer not to have a solid brown or solid white full corner stone, the two diagonal small squares would balance....
You have created a gorgeous quilt. Absolutely beatiful.
I would like to see the whole quilt, or some of the other squares....Great job.
D in TX