The most useful starting point in any outdoor planters buying guide is scale. A planter needs to suit both the plant and the patio: long troughs work well against walls or along decking edges, while round and square pots carry more visual weight and sit best in corners, by doorways or in larger open spots. On smaller patios, narrower shapes generally feel tidier and less intrusive.
Drainage matters just as much as looks, particularly in the UK where heavy rain can leave compost waterlogged in no time. Check whether the planter has pre-drilled holes, removable bungs or a built-in reservoir, and bear in mind that very large containers become heavy once filled, so it is worth deciding on the final position before you plant.
Depth is easy to overlook. Shallow planters are fine for herbs, bedding and trailing plants, but shrubs, grasses and small trees need more root room to stay healthy and to avoid drying out too quickly in warm spells.