They both work great in my opinion. I find that backer rod is the easier of the two to use, especially in the straight walled cases. Just buy a size that is slightly larger than the case size and cut a length about 1/4 inch longer than the air space you have in the case. Stick it in, seat the bullet, and done. With Dacron it's harder to get the same amount measured out so you can have consistency from round to round. I think the backer rod is cleaner on the range as well. The dacron can make kind of a mess.
