I cannot address how they will grow in your area. The maximum height for plants listed at many sites and on plant labels is the expected height at 10 years. Older plants may exceed that height, so sometimes the range varies from source to source depend upon the criteria used (10 years vs. actual maximum). While both can exceed 8' eventually, 'Elfin King' is considered the slower growing, and more compact, of the two. Sometimes it slows down considerably after reaching 5-6'. Due to its slower growth rate, it would be much easier to maintain at 7-8' should it reach that height. So I would recommend Elfin King for that reason. In comparison, I planted a 10" 1 gal. Compacta about 1-1.5 years ago. I just measured and it is already 45" tall!

I agree with the previous poster - 'Elfin King' would be expected to grow slower, remain more compact than 'Compacta'. In my climate, with plenty of water, both will exceed listed height/width in time. And not all that much time :-)) It is important to note that woody plants - trees and shrubs - will always keep growing until they die. Growth rate may slow down significantly once the plants are well established and fully mature but they will continue to grow. Some pruning may be required eventually if they exceed your specific size requirements and due to its more compact habit, 'Elfin King" would be expected to respond to this a bit more easily than 'Compacta'. IMO, I consider 'Compacta to just be about a 2/3 scaled version of the standard Arbutus unedo. as it can easily reach 10-12 feet or more in my climate. Quite a lot more small tree-like than shrub-like.

'Compacta' seen up here are an apparent variable (flowers not even all the same degree of pink or white, right within the same block) seed strain that does not conform to at least one printed account of the selection being an essentially flowerless dwarf.

Since the 'Elfin King' may instead have been kept true, and it is supposed to be small-growing if you can get that one go with it. An example I planted myself recently has been comparatively restrained so far. But does look like it is going to reach 4 ft. sooner than 10 years. Maybe quite a bit sooner.

A similar example of a standard item on the market up here that has not been kept uniform is Viburnum tinus [Spring Bouquet] = 'Compactum'. Different cultivars sold under the name here now do not even have the same leaf shapes or levels of glossiness between them.

If you are interested at all in destination nurseries with exceptional selections of choice kinds of woody plants you have one in your area:

http://sonomahort.com/

Off-topic but if anyone was thinking about planting arbutus, you might also be interested in Chinese Bayberry (Myrica rubra). The shape of the fruit is very similar to arbutus unedo, but the color and taste is more like blackberry. It's very popular in certain parts of China and Southern Japan. You could plant one Chinese Bayberry and one arbutus for variety.