The choice of planter is very much a matter of personal taste. However the correct scale and shape are of the utmost importance for creating a pleasurable effect.
A planter with a larger volume of growing medium will dry out less rapidly and will be able to sustain larger more vigorous plants.
Always look for planters with large or heavy bases to give stability. When first planted most plants will be fairly small. Don't forget to allow enough space for them to grow.
Some planters can be very heavy. Do you want to move them around?. Should they have feet or wheels to facilitate moving the planter.
Make sure there are drainage holes in the container.
The soil should be able to breathe?.
Breakable: Use your judgement. Be careful of clay pots freezing and splitting.
In colder areas of the country a planter that retains the heat better will prolong the growing season. If you have bulbs they will over winter better and your growing season will start earlier.