I know the difference in the plants, but how are they related ?
A good explaination is in The Gesneriad Page. I will put a link on the bottom.
Here is a link that might be useful: The Gesneriad Page
The botanical relationship of these two subgenera of the genus streptocarpus is still not well understood. I think most of us expect that at someday, a taxonomist will separate the two into different genera. Saintpaulias are closest to streptocarpellas, which also complicates things. I fear, a scientist will conclude that saintpaulias should be submerged within the streptocarpellas even though they don't seem to hybridize.
Another way to understand these two groups is to learn about their native habitats. The strap leafed rosulate and unifoliate streps come from South Africa, extending a little to the north, such as Zimbabwe and perhaps some surrounding countries. Some little known species I believe, are also found in Madagascar. Streptocarpellas come from East Africa and Madagascar. But, most of the streptocarpellas that we grow are from the East African species. Many of these types are found in nature in the same mountains as the saintpaulias. The Madagascar species of streptocarpella all seem to have very tiny flowers, while the East Africans have larger flowers, hence their popularity in cultivation. To a large extent, the hybrids come from two species, saxorum and stomandrous with 'Concord Blue' and 'Good Hope' being a direct cross of these two species.
I am probably making mistakes left and right in my explanation. I hope John B. doesn't find too many mistakes, but I welcome him to correct me.
Another thought: What unites these two subgenera of the genus streptocarpus is seen in their shared name, which means "twisted seed pod". There are other old world gesneriads with long skinny bean pod shaped seed pods. But, they don't have that twisted seam like the streps.
Also note: the names are spelled with an "o" not an "a" as in the subject line: streptocarpus and streptocarpella.
Think of these two groups as the "Twisted Sisters" ;)