Depends on what you are sowing and the size of the seed. Generally the smaller the seed the easier it is to sow in trays. Anything big enough to handle may be sown in cells. Transplanting does give the plant a check in growth and some things hate it.

thanx owdboggy

I personally prefer to sow into cells, seedlings can then be left without any handling at all, once they are big enough the individual cells can be moved on into 3" pots, then to garden, they appreciate the tender handling. I've always had great success using this method, for finer seeds, i tend to sow thinly into 3" pots, and again into larger pots or straight into the ground or hanging basket.

Paul