Bareroot trees (dormant, with no soil around the roots) are available in winter, in the widest selection, from the most sources and at the best prices. Especially if planted early, they are well-established before any hot weather. Bareroot trees may be topped, limbed and shaped as the grower chooses.
Container trees are conveniently available in spring, but at a higher price than bareroot, and all varieties and variety-rootstock combinations may not be available. For the first couple of weeks, actively growing trees planted from containers must be watered frequently, sometimes daily, til their roots establish in the surrounding soil. If planting in late spring from containers, the trees may not be well-established before early hot weather: be sure not to miss an irrigation before or during an early hot spell.
Planting well-grown containerized trees in the fall can be especially successful. By fall, the trees are well-established in the container and there should be little or no transplant shock from a combination of jostled roots, missed irrigation and/or hot weather as might occur in late spring.