PDH link is used for low capacity. A PDH link has from 4E1 up to 16E1s or even 32E1s depending on vendor.
While SDH link is used for high capacity. An SDH link has from 1xSTM-1(63E1s) up to 8xSTM-1 (8x63E1s), also depending on vendors.
Usually SDH links are used for backbone network, while PDH links are used to connect BTS with Backbone sites or BSC.