How should I decide?
Apart from the more popular cultivars which can be found in almost every well-stocked nursery, there is a considerable number of other cultivars and many new hybrids. The 'Plantfinder' (English) or the 'PPP Pflanzen Index' (German) offer a good survey where these are available. Meanwhile, Agapanthus can be found now and then in nurseries on the Continent.
What can be said in favour of the Evergreen Agapanthus? They are simply marvelous. The foliage in its large container is very decorative, and it is amazing to observe how the buds are slowly rising on their stems and open to flowers of magnificent splendour.
A sunny site in the garden or on the terrace are sufficient in the summer. But in the wintertime, you have to have a light, cool (5 degrees) place for them. This is rather hard to find in centrally heated houses. Apartment houses with their cool staircases are a better bet. The neighbours maybe won't object to have the landings turned into miniature coldhouses. Another problem is posed by the size and weight of the containers. You may need some help to get them down to the cellar (light!) or up the staircase.
Deciduous Agapanthus are often quite a bit smaller. The satisfaction to see a plant flower can be achieved with a relatively small container. It needs less strength to carry the plant inside. As there are no leaves (the plant will shed them anyway once it is in the darkness), a dark cellar or windowless garage will do. The temperature should be 5 degrees as well.
The decision what to buy will be largely dictated by the regional climate and the availability of a suitable spot to keep them over the winter months.
On the Continent with its long and cold winters, Deciduous Agapanthus have a fair chance of survival in the garden, provided they get a sheltered spot. A good winter protection is essential, they do not need more care otherwise. Evergreen Agapanthus have to be taken inside, read more on this topic in the 'winter protection' section. Only in very sheltered regions, as the English South Coast, do they thrive outside year round.
If it is possible to find out which cultivars have proved a success in the respective areas, this should help to make the decision process easier.