Standing in a garden centre aisle with a seed packet in one hand and a punnet of seedlings in the other is a genuinely common moment of indecision. This site's seed-vs-seedling cost, time and variety trade-offs article covers the full reasoning behind the decision; this one is the short, crop-by-crop verdict for this weekend, plus something that article doesn't cover at all: how to actually pick a good seedling once you've decided to buy one.
The quick verdicts: buy seed for carrots, dwarf beans, climbing beans and peas, since all four are sown direct anyway and there's rarely a seedling alternative worth looking for. Buy seedlings for tomatoes and capsicums unless you're planting early enough in spring to raise your own indoors, and for celery almost without exception, it's a genuinely fiddly, slow-germinating crop from seed. For lettuce and beetroot, either is fine, so let convenience decide. For onions, seed rewards patience with a long autumn-to-late-winter sowing; seedlings or sets are the fair substitute if you've missed that window.
Once you've decided to buy, the punnet you pick matters almost as much as the decision to buy one. Look past the healthiest-looking foliage on top and check the base of the cell first: roots circling or poking out of the drainage holes mean the seedling is root-bound and has been sitting in that punnet too long, which shows up later as a plant that's slower to get going once it's in the ground. Favour short, stocky, deep-green seedlings over tall, pale, stretched ones, a leggy seedling has been reaching for light in a crowded tray and rarely catches up to a stockier one planted at the same time. Skip anything already flowering or setting fruit in its punnet, that's a plant under stress from being pot-bound too long, not one getting a head start. Turn a few leaves over to check for aphids, whitefly or eggs before they come home with you and spread to the rest of the garden.
A nursery-bought seedling has usually spent its life under some degree of shade cloth or greenhouse cover, so it benefits from the same hardening-off treatment as one you've raised yourself: a few days outside in a sheltered spot before it goes into open ground, rather than straight from a covered garden centre display into full sun and wind on the same day.
If you're still unsure after checking the crop itself, default to seedlings when you value speed and forgiveness, and default to seed when you value cost, variety, or the crop is one that's normally direct-sown anyway. Getting this one decision right per crop matters more for a beginner's success than almost anything else in the garden that weekend.
If you're still unsure after checking the crop itself, default to seedlings when you value speed and forgiveness, and default to seed when you value cost, variety, or the crop is one that's normally direct-sown anyway.