1.Call transaction is an immediate process where as session 
is a delayed process.
2.Exception handling is automated in session where as in 
call transaction method we have to handle it manually by 
using BDC_MSG_COl.
3.Locking is possible in session method where as in call 
transaction method it is not possible.

Session Method :
1.Asynchronus Processing
2.Synchronous Database update
3.Session Can not run in parellel.
4.A bathch i/p log is generated for each session

Call Transaction
1.Synchronous Processing.
2.Synchronous & Asynchronus Database update.
3.No bathch i/p log is generated .

session method:
1.Asynchronous processing i.e we can process multiple
transaction codes in a single program.
2.Synchronous data base updation
3.Error log created.
4.No significance for sy-subrc.
5.It is slower than call transaction method.
6.We cannot execute this method directly.

call transaction method:
1.Synchronous processing i.e we can process single
transaction  in a single program.
2.Synchronous and Asynchronous database updation.
3.Error log cannot be created.
4.Significance for sy-subrc.
5.It is faster than session method.
6. We can directly execute this method.