Answer
The P-N bond is more polar.
For P-H: Phosphorus receives the partial positive charge, and hydrogen receives the partial negative one.
For P-N: Phosphorus 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 phosphorus and nitrogen (3.0 - 2.0 = 1.0) is greater than the same for phosphorus and hydrogen (2.1 - 2.0 = 0.1).
Since that difference is proportional to the polarity of the bond, the P-N will be the more polar one.
For P-H: The less electronegative one is phosphorus, so it will be the partial positive atom, and hydrogen will be the partial negative.
For P-N: The less electronegative one is phosphorus, so it will be the partial positive atom, and nitrogen will be the partial negative.
