Every spring we get the same question from homeowners in Forest Grove and Cornelius staring at a muddy backyard: should we seed it or sod it? There's no wrong answer, but there is a right answer for your situation.
Sod gives you a finished lawn the same day. It controls erosion immediately, which matters on sloped lots and new construction, and it can be installed almost any month the ground isn't frozen. It costs more up front, and it still needs daily watering for the first two weeks while roots knit into the soil underneath.
Seed is cheaper per square foot and gives you more choice of grass varieties, including blends that handle shade under big firs. The trade-off is time and attention. Seed needs consistent moisture for three to four weeks, a season or two before it handles heavy use, and it's vulnerable to washout in a hard rain and to birds and weeds in the meantime. Fall is the best time to seed on the west side; spring works if you stay on top of watering once the rain stops.
Either way, the preparation is the same and it's where most lawns fail. Strip the old turf and weeds, fix the grade so water runs away from the house, loosen and amend the soil, and make sure irrigation covers every corner. Get that right and sod or seed will both give you a lawn you're glad to come home to.