I agree that there is a recognizeable gap between the two, but it seems to me that this was more of a recognition that any channeling Egwene used would be at a disadvantage against Mesaana. I think Mesaana is stronger than Moghedien, mainly because Egwene became familiar with Moghedien's strength level and was used to the idea that Nynaeve was more powerful - her reaction was less to do with terror and more of a calculated retreat.