Sat 08-Sep-12 10:24 AM | edited Sat 08-Sep-12 10:25 AM by mklass
If you buy the same make and model lens, whether as a kit or separately, they are exactly the same.
If you are buying from someone who tries to up-sell you to a "better" lens of the exact same model, for instance by telling you the "better" lens has a metal mount ,run away. That seller is trying to scam you and shouldn't be trusted.
One thing to be aware of is that there may be older models of lenses, or apparently similar lenses that are truly different models. The Nikon 70-300, for instance, come is a model with VR and one without. The VR model is a very good lens and cost about 4 times as much as the non-VR model, which isn't a very good lens.