Hi,
BDC and BAPI both are equally good for Data MIgration but BAPI is always advisable as its is faster that BDC transfer.
BAPI directly update data on database server without using any screens but in BDC we need to deal with Presentation Server that make the transfer slow.
BAPI uses direct structure of table to update data in database server while in BDC we need to first use BDCDATA structure to contain the structure that have program name, etc. and while using BDC if you used Session method that you need to run the session manually that also make the data transfer slow.
