The two constructions are interchangeable in meaning and use, except for one small detail: The phrase "each/some of which" requires a verb, while "each/some of them" doesn't:
1.1. The paragraph is composed of sentences, each of them nominal.
1.2. The paragraph is composed of sentences, some (of them) nominal, others (of them) verbal.
2.1. The paragraph is composed of sentences, each of which is nominal.
2.2. The paragraph is composed of sentences, some of which are nominal, others (of which are) verbal.
(The phrases in parentheses are optional. Of course, if you keep the second in the sentence with two, you have to keep the first as well.)
2.1. and 2.2. use adjective clauses (the "which" clauses that follow the first comma). 1.1. and 1.2. make use of a different sort of technical construction than in 2.1. and 2.2. I think they are a combination of prepositional and participial phrases with words omitted as understood, but I'm not sure:
1.1. The paragraph is composed of sentences, (with) each of them (being) nominal.
Whatever they are called, they are very standard, and they work equally well.