I assume your asking which material?

I am just starting my first SOF with a Yost frame. Went through the same questions. We don't have Douglas Fir around here so I have not worked with it. But I would think either one would work just fine. Your might be making it little heavier with the fir instead of cedar.

I just scarfed up some strips last night for mine. Can't find good cedar so I bought spruce (who knows what it really is) Ripped to width and cut out the knots. Then just scarfed up long pieces.