I prefer W&N batting. I have used low loft poly and it is ok. I have also used flannel; just be sure to pre-shrink it. I have not had any problem with tall items falling over when I set them on a runner with a batting, but the seams on runners without batting create more high and low spots so things don't sit right.
I sew scraps of W&N together to make batting the right size for the runner I'm making. It's a great way to save money and works just as well as a full piece of batting.