First of all: Regardless of construction materials (or where you get them), a Dyson Sphere would be
useless because GRAVITY is impossible on the inner surface of a Dyson Sphere. {See the
Dyson Sphere FAQ, or even the
Wikipedia entry on Dyson Spheres.}
Secondly: Building
either a Dyson Sphere or a Ringworld requires the ability to disassemble whole planets, the ability to convert matter into energy (and vice versa), and the ability to rebuild matter on an atomic level. So when you allude to "benefits" from building such a thing, you're talking about "benefits" that simply do not exist for anyone capable of building it. Think about it: What exactly are those "benefits?"
Capturing a larger percentage of the energy output of a star? Why would you
need it? If you have such godlike technology that you can convert matter into energy, then
you already have an endless supply of energy.
18440 times the surface area of Earth? Again, why do you
need it? If you can disassemble whole planets, and can control matter with the ease necessary to fabricate your hypothetical (and impossibly dense) Dyson Sphere & Ringworld building material, then why would you waste it building anything that's tethered to the gravity well of a star? You could far more easily build
mobile planets orbited by their very own artificial micro-stars.
I repeat: Nobody who
could build one would
need to build one. It's just irrational.