As far as true advantages, the only one I'd really call a pure advantage by brand is that Pentax will promise their lens will work and continue to work and will support their lens if they make some kinda change to the camera body. Since Pentax is the king of backwards compatibility, I don't really know how much of a problem that'd really be anyway.
In general, Pentax is known for having extremely good lenses, but it's been my experience (probably due to my limited budget) that Pentax and the off brands just don't have the same choices available at the same price points. Because of that, I don't that a sweeping generalization of which lenses are better is all that helpful; I think you'd be much better off determining what function you want the lens to serve and your price point, and then reading reviews of those specific lenses to choose.
I don't know if there really is a good general rule.
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But Mom, Pentax IS rebellious
Pentax K-7, K20D
Pentax SMCP-FA 35mm f/2.0 AL -- Pentax SMC 50mm f/1.7 -- Pentax DA 50-200mm f/4-5.6 ED -- Sigma 28-70mm f/2.8 EX DG IF Aspherical -- Pentax DA 18-55mm f/3.5-5.6 WR
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