Which boundary curves should I choose in Stokes' Theorem?

Question

I have to verify Stokes' Theorem to the field $$F=(3z,4x,2y)$$ and the surface $$z=10-x^2-y^2,\quad 1\leq z\leq9$$

For $z=1$ and $z=9$ I get two circles. I'd like to know which one I should choose to evaluate the line integral.