In what way, the sculptures at Sanchi Stupa were different from Bharhut?

The best examples of Buddhist stupas are found at Amaravati, Sanchi, Barhut and Gaya.
Around 100 B.C., a great stupa was made at Bharhut, in the eastern part of present-day Madhya Pradesh. Bharhut stupa was initially built by Asoka and was later improvised by the Sungas. The railings of the stupa and its one surviving gate are at the Indian Museum in Kolkata.
This is the earliest stupa railing to have survived. Unlike the imperial art of the Mauryas, the inscriptions on railings of Bharhut stupa show that the reliefs and figures were donated by lay people, monks and nuns. Thus, it is one of the earliest examples of Maurya popular art. The railings contain numerous birth stories of the Buddha’s previous lives, or Jataka tales. The Bharhut stupa represents the aniconic phase of Buddhist art. Buddha has been represented in the form of symbols.
Sanchi in Madhya Pradesh is a site of three stupas out of which the first and the oldest Great Stupa was originally commissioned by Asoka. The ‘Great Stupa’ at Sanchi is the oldest stone structure of India and was originally commissioned by the Ashoka in the 3rd century BC, vandalized by Pushyamitra Sunga and rebuilt by his son Agnimitra and again improvised by Saatavahana kings.
Thus, this stupa represents a testimony to the Maurya, Sunga as well as Saatavahana art in India.
Its nucleus was a hemispherical brick structure built over the relics of the Buddha. It has upper as well as lower pradakshinapatha or circumambulatory path. It has four beautifully decorated toranas depicting various events from the life of the Buddha and the Jatakas. Figure compositions are in high relief, filling up the entire space, thus showing remarkable improvement from the Bharhut.
The narration in the inscriptions which was concise in the Bharhut has got elaborated at Sanchi. However, the Symbols continue to be used representing the Buddha.  The earliest and largest stupa found in Sanchi is ascribed to Asoka.