i would say that a major difference in the two species are bloom time; fall for the sasanquas and winter/spring for many japonicas.
also, the japonicas can have blooms that fall within the range of all categories including single, semi-double, formal double, peony, anemone, etc.
in my experience, flowers on japonicas for the most part can be larger than those of sasanquas.
as for growth habit, i have seen small japonicas and large sasanquas.
here is a link with some basic information about them.

Here is a link that might be useful: camellia culture

Most japonicas typically have larger leaves than most sasanquas.

Another difference is when flowers pass their prime: in japonicas the flower falls as a whole, and sasanqua flowers normally drop petal by petal.

The growth habit as far as plant appearance, japonicas usually look much more formal and stiff, while sasanquas can be more open and loose.

Thanks~

And it's a law that you have to plant too many of each...

I think I know that law. It is definitely observed to the letter in Charleston!

I will pass this law on to the nursery owners. They will be thrilled!

Sasanquas can be planted in a wider variety of exposures than Japonicas. Provided reqular watering they will thrive in full sun to full shade, whereas Japonicas tend to be more of a morning sun, afternoon shade lover. Japonicas have larger leaves and tend to be somewhat slower growers than Sasanquas. As for size of each it will depend on the cultivar selected.

According to japanese texts, as a general rule, sazanka (the japanese name for C. sasanqua) fall off petal by petal and bloom in autum.

On the other hand, tsubaki (japanese for C. japonica) fall off as a single flower and bloom in the spring. This is why samurai considered tsubaki unlucky...it resembles a person's head falling off.

Some laws are good ones to have!!

They are different genetically. Most Sasanqua has 90 cromosome number and most Japonica,30.

Sasanqua species is very close the species of C. oleifera. They all have good root system and stand more sun light. Petals are readily to break loose to fall.

Sasanquq has a typical scent, not all pleasant odor.

It is a welcome species to flower early before real cold air.

Sasanquas are willowy and bronzy green, sun-loving. Japonicas are comparatively dense and stiff, need shade to look their best - something that is indicated by their deeper green foliage. Their usually unscented flowers often shrivel and cling to the bush. Otherwise, their fallen petals make an amazing amount of litter. Eastern (morning sun) exposures are NOT suitable for them, actually, as the likelihood of frost injury to the flowers is high in such places.

Sasanquas generally have smaller leaves, a looser habit, bloom earlier, drop petals everywhere and grow taller.
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This camellia grows in full sun and has a tea fragrance when you walk past it.

C. sasanqua is in riotous bloom right now here on Kyushu. It is grown everywhere, usually sited in full sun positions. C. japonica grows in the wild here in large numbers. It seems to bloom from January on into mid-spring, perhaps until May. It is great to see both flowers once they fall. Sasanqua leaves wonderful piles of pink petals and japonica litters the forests with perfect flowers...I'll never forget seeing for the first time all these lovely flowers scattered over fresh fallen snow in the mountains. Truly magical. PF

Where would this hedge be located? In how much summer sun? What is the USDA Zone there?

IMO, Sasanquas are much more tolerant of different conditions than japonicas.