As per efi thread :
Calyx teeth are deltoid in S. trifida (not filiform) and as long as tube (longer in S. delphinifolia). The flowers are smaller (1 cm) and of uniform pink colour as compared to S. delphinifolia. S. delphinifolia occurs on dry grasslands.


Sopubia comes under the group where corolla is not bilabiate; tube enlarges at throat. Perfect stamens 4. Calyx ribbed and anthers in pairs 2 celled. (Whereas calyx not ribbed; anthers not in pairs, 1 celled in Rhamphicarpa)
Coming to difference at species level
Calyx teeth filiform, longer than calyx tube … … … … S. delphinifolia
Calyx teeth deltoid-ovate,equalling calyx tube .. … … S. trifida

Sopubia page with images of species in efloraofindia : 1 post by 1 author.
Pl. go through Sopubia page with images of species in efloraofindia.