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The most important thing: look at your soil before you decide. If the ground is compacted or you've got a lot of weeds, even the best sod won't help - you'll need to do some serious prep work first.
Seed is cheaper and gives you more control over soil preparation. You can see exactly where there's compaction and loosen it up. The downside: it takes longer (several weeks before the lawn can handle foot traffic) and germination is more finicky - too dry, too wet, or bad weather and you end up with bare patches. Sod costs more, but you get an instant finished lawn and can use it after just a short time. The catch is you need to lay it down properly, or else it'll buckle or dry out.
For me personally: if I've got the time and the soil's okay, I'd go with seed. A few weeks of patience saves money and you don't do this that often anyway. But if you've got a large area and want to be done quick, or your soil's really problematic, then sod is the better choice - even though it'll cost you.
It really depends on what standards you have and how much time and money you're willing to invest. Soil analysis is of course important, but to be honest it's not the deciding factor between the two options. Even with mediocre soil you can get a decent lawn with seed - it just takes patience.
Seed is way cheaper, and if your soil isn't totally messed up (compacted, full of weeds), something will grow. It just takes 4-6 weeks until the lawn is usable, and during that time you have to water regularly - even if it rains, because the germination phase is critical. Sod is instantly ready and saves you the waiting time, but it costs a lot more. It makes sense if you need a neat lawn quickly or if the area isn't huge.
What I think is important: no matter what you go with, prep the soil first. Get the weeds out, loosen up compacted spots, top it up if needed. With seed you just have the costs for prep and seed together, with sod you skip the germination phase but pay for the prep plus the more expensive sod. If your budget is tight, go with seed. If it needs to happen fast, get sod.
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