Given a full season and patience, seed usually wins. Given a deadline, sod is the only option.
Sod
Instant, holds soil on a slope immediately, and can go down most of the season. Costs several times more per square metre and needs serious water for the first three weeks — miss that and it shrinks and gaps.
Seed
Cheaper, wider variety selection, and roots that develop in place rather than being transplanted. Needs a consistent seedbed, consistent moisture and time, and it competes with weeds while establishing.
Timing
Late August into September is the best seeding window in the GTA: warm soil, cooler air, reliable moisture and weaker weed competition. Spring seeding fights crabgrass all summer.
What both need
Real topsoil on a prepared grade. Sod laid on compacted construction fill is money spent on something that will be brown by August, and seed on the same base does not establish at all.
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