Horseshoe or Seminar Circle
Both plans seat 20. Whether the discussion needs a writing surface.
Horseshoe
Choose the horseshoe when students talk and still need a desk in front of them.
16 by 8 grid, 4 rows.
Seminar Circle
Choose the seminar circle when the hour is talk, and paper on a desk would pull eyes down.
8 by 8 grid, 6 rows.