Viharas were for the purpose of living, Chaityas were assemblies for the purpose of discussions. Further, Chaityas were with Stupas, Viharas did not have stupas.
The Gandhara School of art was developed in first century AD along with Mathura School during reign of Kushana emperor Kanishka. Both Shakas and Kushanas were patrons of Gandhara School, which is known for the first sculptural representations of the Buddha in human form.
EXAMIANS