Generally, a transplant with four or five leaves can be planted at the same time as seed, so you get a head start on making large bulbs. An onion set is a small bulb, stored from the previous year's onion crop. A transplant is a plant between 10 and 20 weeks old that has not been through the bulbing process. An onion is a biannual, which means it has two lives. In its first life it grows from seed to set, and in its second life it produces a larger bulb.
Sets are usually quite pungent and only good storage types can be kept for next year.
A transplant is much less likely to bolt or go to seed than a set. Almost 70 percent of all onions are yellows, and there is a good reason for that. Yellow onions tend to be easier to grow than whites or reds. They generally have a tougher skin and heavier foliage. They also are more susceptible to sunburning.