I would go with the best planks. Not sure either is ideal, but both would work. SOF have been built from recycled shipping pallets with lots of scarfing. One of the most important aspects of your gunwales besides overall soundness will be even bend ability. Therefore, given soundness, I would look for the most even grain. Hemlock can have a lot of variability. For reason of bending, we often split the gunwales out of the same plank rather than use two separate boards. I would watch for brittleness in the cedars, especially when it comes to rib mortises.