I’ve never even heard of the term “double ply”. Single ply refers to a system that, when done correctly, is virtually seamless so that water may not penetrate it’s membrane. And then you’ve got a 4 ply BUR (built up roofing, or in lehmanns, tar and paper). Both are good options, but are only as good as their installers. A warrantee, too, is only as good as the person offering it. Single ply roofs will most definitely provide a stronger warrantee in my opinion, but unless its inspected by the manufacturers rep. you are still just trusting the craftsmanship and honesty of the roofing company. I think that you will find BUR to be cheaper than single ply roofs, but less appealing to the eye, and a dwindling work force of skilled installers for it. I think when done correctly they will have a similar life span.