Answer
The C-O bond is more polar.
For C-O: Carbon receives the partial positive charge, and oxygen receives the partial negative one.
For C-N: Carbon receives the partial positive charge, and nitrogen receives the partial negative one.
Work Step by Step
According to the picture 6.8 (Page 263), the difference between the electronegativity of oxygen and carbon (3.4 - 2.4 = 1.0) is greater than the same for nitrogen and carbon (3.0 - 2.4 = 0.6).
Since that difference is proportional to the polarity of the bond, the C-O will be the more polar one.
For C-O: The less electronegative one is the carbon, so it will be the partial positive atom, and the oxygen will be the partial negative.
For C-N: The less electronegative one is the carbon, so it will be the partial positive atom, and the nitrogen will be the partial negative.
