The trick here? Both are made by the same company: Luxottica. Both have generally similar construction quality, and thus both will likely last just as long. Don't be fooled here -- the differences between these two pairs of sunglasses are minimal, the only difference being that the D&G glasses are more expensive/flashy, and the Ray Bans are more classic.
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Gap products go on sale so often for a reason. They have a half-life of approximately 8 months. If you buy 8 gap shirts, you can expect to have 7 decay after 2 years of gentle use. They still make a profit at $5 a shirt; that ought to tell you something about the quality of the material and workmanship.
That said; AmAp, despite being made in the good ol’ USA, is also remarkably low quality.