Transferring large volume of data from SAP ECC to SAP XMII

Hi all,

What will be the best method to transfer large volume of data from SAP ECC to SAP XMII. We need to transfer the batch and inventory details from SAP ECC to Oracle using XMII. The data size ranges upto 2 millon. Which method of communication(BAPI/IDOC/Web Services) will provide the optimum performance? Also kindly let know some ideas of transferring this large volume to XMII.

Thanks