Plain edge for sure! But then, I'm biased against combo-edge knives. I have yet to find a task for them that a fully-serrated or fully-plain edge blade can't do just as well if not better. Also plain-edge knives are a lot easier to sharpen. 
I second 3mptin3ss- easily my favorite Spyderco design. The only thing that comes close is the Stretch CF, in my opinion.

I second 3mptin3ss- easily my favorite Spyderco design. The only thing that comes close is the Stretch CF, in my opinion.