Neither Sigma or Quantaray  is much of a brand name. yes Sigma probably
makes the lens. Try to let Ritz test the lenses. With any Sigma lens
there will be a sample variance of quality, buy which ever is better,
don't buy one if niether is better than your existing lenses, which
could happen.

Tom

I'll tell you why the Quantaray is cheaper. Ritz Camera contracts with igma
for these lenses the same way WalMart squeezes their vendors. Make it
cheaper.
Bob

The whole issue of lens quality is very perplexing. On the one hand you have
some who would claim that anything badged as Nikon or Canon is superior to
anything badged Sigma. Having seen the dogs that are tagged Nikon and Canon
at premium prices I can only shake my head in wonder. The Nikon 55-200 is
one of the worst lens I have ever seen and is but a recent addition to the
Nikon line-up. The Canon dSLR kit lenses are the only lenses I have ever
seen used to demonstrate chromatic aberration in every demo of how to
correct this problem in Photoshop. How could you possibly go worse with a
Sigma? Build quality? Take a close look at the least expensive Canons and
realize the volumes at which these things are cranked out and their target
markets-BestBuy and Costco do not cater to discriminating
photographers--because that is a factor in determining manufacturing
quality. The vagaries of Asian corporate and manufacturing structures are
such that it is highly likely that lenses from different vendors are
manufactured at the same facilities. There are instances where it is
entirely unclear if the source of a name brand lens is not a secondary
manufacturer like Sigma or Tamron or even lower on the food chain.
Caveat emptor. Just don't be empty headed about the whole issue.