Root ball plants are field grown in open ground and then lifted during the dormant season with the soil still attached around the roots. The root system is wrapped in hessian or a biodegradable netting to hold the soil in place. Because the roots are kept within their natural soil there is less disturbance compared to bare root plants and they establish very well once planted. Root balls are typically available from late autumn through to early spring and are ideal for larger, more mature trees.
Container grown plants are grown in pots rather than in the ground. Their roots develop within the compost inside the container which means they can be planted at any time of year as long as the ground conditions are suitable and they are kept well watered. They are generally easier to handle and move around, especially for smaller sizes, and offer more flexibility with planting times.
In simple terms, root ball plants are field grown and seasonally lifted which often makes them better value for larger sizes, while container grown plants are pot grown and available year round, offering more convenience and flexibility. Both establish well when planted correctly, so the right choice usually comes down to the time of year and the size you are looking for.