I think I am wrong about the definition of "Flat fan" tip. After reading through the nozzle information linked by Green Dr. It is clear that a flat fan is tapered at the edges--not a sharp cutoff--as I thought. The flat fan is designed for boom spraying with the edge of the pattern overlapping for full coverage.
My opinion is that a circular nozzle pattern is best for back pack use or for any type of back and forth motion. Just do not delay much as you reverse motion at the far right (and left). And for spot spraying: use a sweeping motion for individual weeds, (if you want to avoid overdosing the spot).