You get better conversion if you transfer edge to ITC to Marriot.
Ex: Let's suppose you have to transfer 500 edge reward.
1) Edge to ITC -> 400 (5:4)
ITC to Marriot ->600 (2:3)
So, effective conversion, 500 Edge -> 600 Marriot points.
2) Edge to Marriot -> 400 (5:4)
So, you get more marriot points if you do first step but obviously it's time consuming process.