There's a difference?
A significant difference, that is. I've bought a few 100 yd spools of 20# dacron polyester, but usually I buy a 1200 yd spool via mailorder because the bulk price is less expensive. I don't think there's a significant difference in quality between the brands - although Bob indicates a difference in diameter for some. I suspect that most of the "brands" are manufactured at the same few factories that produce this product.
I did recently pick up a 300 yd spool of 30# for the first time, not because I think I need that strong a backing - my tippets are only 8 or 10# test. I got the 30# because I thought it might be nice not to have to reel on a half mile of backing on my largest fly reel to get the flyline out near the edge of the reel spool.
I've never worried about the 20# backing getting nicked and broken, as it's attached to floating line and when in use is either in the air or on or near the surface of the water, where getting nicked would be highly unusual. In fact, I've got backing that's over 30 years old on some of my reels. I just never think to change it. It spends most all the time in a cool dark place, unaffected by sunlight. Maybe oughta' check some of the older stuff, and see if it's still strong. I bet it is, but if it isn't, my oh my, I'll be buying and reeling several miles of the stuff, as most of my reels and backing have been around a good while.
Sincerely,
Salmo g.