I have the Martelli 17" Roundabout. One of the most useful tool I have bought for quilting. I also have the 10" Sue Dailey one I won as a door prize. I use the bigger one the most. The small size doesn't seem to be as stable to me even when cutting small pieces. I usually have the small size next to my machine and the large one on my cutting table. Most of my cutting in on the rotating large mat.
The small size is of course easier to pack and tote.