More important question: Which contractor did you feel the most comfortable with and which has the best work history in your area -- have you checked BBB and something like Angies List? Who has the best relationship with their wholesaler/supplier and can get you support if you need it in case of roof damage?

Owens Corning and GAF make comparable products; the Owens Corning warranty is longer, but neither warranty is good unless the installer installs it properly with the proper number of nails at the proper spacing. Specifically, the two products that you're looking at are almost identical. One difference besides the warranty is that Owens Corning has a constant bead of sealant on the backside of the shingles to seal them to the shingles underneath; if you're in an area that gets significant amounts of wind-driven rain (such as a coastal area) to the point that you will be testing the wind resistance of the shignle, you will PROBABLY want to choose the GAF shingle because the Owens Corning shingles sometimes have problems draining after water has blown up underneath them.

Remember that it isn't the shingle that matters; the shingle really just protects your roof. Your roof is actually the tar paper and other membrane (such as Grace Ice & Water Shield) that goes on under the shingles. Those products should be of the highest quality and have perfect installations. Getting good shingles is essential, but it's almost secondary if you cheap out on the underlayments or your roofer installs it incorrectly.

One note that I'm suggesting all homeowners explore: If your attic is hot during the summer, look into the 'Cool Roof' shingles that both OC and GAF produce. They have a reflective substance in with the stone and will help reflect solar energy back out before it has a chance to enter your roof structure.

That's what I've done in similar situations. Cut off the top half of the shingle (using the piece without the glue line) and 2-4 nails right through your last shingle. Cover the nails with a drop of roof cement.