are the quads worth 15$ more? if so, why?

I have both (2 pair of quads, 1 Backsaver) and IMO the quads are more heavy duty. They also have an elastic section that gives the harness a little "bounce" and absorbs some shock when you have a full bag. I think the foam used in the quads is better. So yes, they are worth $15 more.
Backsavers are a great deal for the price, especially if you don't have quads, or want so save a few bucks.

Spend the extra....the shock itself makes it worth it!

Out of all the DG equipment I've owned, nothing has exceeded my expectations more than my quads. I got a new pair a year and a half ago because the ones I'd been wearing for 5 years were starting to look ragged. I haven't broke out the new ones yet because the old ones are still going strong.