when to use call tranction & call session methos in BDC?

Can any body tell me when to use call transction & when to session method in BDC?

In real time which method will we use?

I know call transction is for small amount of data, synchronous processing, Asyschronous update, Return sy-subrc & all these difference.

Tell me exactly when to go for call transaction & when to go for session method?

Thanks in advance.