Session method: it can handle small amount of data as well 
as large amount of data.

          It can handle multiple applications 
simultaneously.

          This method by default has log files. Log files 
is used to store error records.

          It can process the data fore ground and back 
ground because it can handle small amount of data as well 
as large amount of data.

          It processes the database Asynchronously and 
updates the database synchronously.

  Call-transaction Method: with this method we can process 
the data in fore ground only.

          This method compatible small amount of data only. 
user can create log file explicitly using BDCMSGCOLL 
structure.

             It processes the data and updates 
synchronously. 

             It can handle one application at a time.

SESSION METHOD:
                  RERUNNING THE ERRORS,
                  DETAILED LOG,
                  MULTIPLE TRANSACTIONS,
                  ASYNCHRONOUS PROCESSING,
                  SYNCHRONOUS UPDATE.

CALL TRANSACTION:
                  SYNCHRONOUS PROCESSING,
                  ASYNCHRONOUS AND SYNCHRONOUS UPDATE.

SESSION METHOD :
1) Asynchronous Process
2) Synchronous update
3) Multiple Transactions can be handled ( That means u can 
upload the data at a time for more than one T-Code.
4) Implicit Error handling (Means for errors SM35).
5) Huge amount of data can be uploaded.
6) Execute the session on a specific date.(By using hold 
date parameter).
7) Time Requirement is more.

CALL TRANSACTION :
1) Asynchronous process.
2) Asynchronous/Synchronous Update.
3) Single transaction only handled.
4) Small amount of data uploaded.
5) Explicit error handling(That Means all the error records 
move to internal table and validate)
6) Time Requirement is low.

Guys you done a good job.
I would like to add one more point to your stuff.

In Call Transactions we can Handle Dependent Transactions Also.
In Session Method we can't Handle Dependent Transactions.