I think when people say lower quality, they often mean "less support." You see the Genesis promo renders, and they're extremely well-done, and feature tons of things not available in Poser. Hair, clothing, textures, characters (especially ethnically diverse characters). There are a lot fewer Poser products, and the quality of the renders is often poor (not because of Poser's deficiencies).

If Genesis figures are "better," it's because of the ability to easily share morphs, clothes, etc. And geografting.