Square pots or round?

I don't think the plants are bothered what shape pot it is in as long as it is big enough for the length of time it is in it. If you are going to plant out reasonably soon then not so much compost is needed. The article Proserpina linked says that a square pot has about 25% more compost than a round one that would fit just inside the square one, may be a bit more if the sides are nearer to the vertical.
You would waste less water watering a tray of square pots. I use round pots as I got a load and also some purpose trays that a dozen pots fit into for carrying, free from a nursery whose owner was retiring. She said that she used to use a handle that would fit 2 trays on so she could carry 4 trays, 48 3.5" pots, at a time but she didn't have any then.