I have used both the Montbell and Kooka Bay pillows. The Kooka Bay XL size that I have is about an inch more on the thickness than the Montbell. Weighs about the same. Certainly both do the job supporting the knees. Which I frequently use it for.
The fabric of the Montbell is a little more soft and somewhat more quiet. How do you like those subjective terms I put a fleece cover on the Kooka Bay, just feels better and well, makes it quiet. My wife is a ground sleeper and likes the pillow with the cover on.
Quality wise both are very well made no issues with either, many nights on the trail.