I have a whole bunch of reasons I will share in the next few weeks but lets start with a really simple one.

Challenge: Import 10,000 customer records into GP.

Results:
SmartConnect: 1 minute 30 seconds
Integration Manager: 4 hours 40 minutes

Those numbers just look crazy so we simplified the test and tried again.

Challenge: Import 100 customer records into GP.

Results:

SmartConnect: 2 seconds
Integration Manager: 2 minutes 51 seconds

SmartConnect WINS!