When deciding between contour plowing and terracing what might a farmer consider?
That really all depends on where that farmer lives, what terrain he/she has to farm off, and the costs of contour plowing versus terracing. Some areas are not suitable for crop farming no matter what the options are, so the answer to that could be neither contour plowing nor terracing.
What is the relationship between the relief of an area and the contour interval on a map of the area?
They both run perpendicular to the slope of the land in that spot. For example, if one particular spot slopes north, the contour runs east/west. The difference is that terraces are normally built-up areas of soil which are intended to catch runoff water in a basin-shaped area. Contour plowing is not built-up above the rest of the surrounding area.