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Is the lack of AF Motor on the Nikon D5100 an important factor?

I am just starting up as an amateur photographer to venture into DSLRs. Being on a tight budget, I am currently considering Nikon D5100 and Canon 600D, with preference towards Nikon due to its better image quality, focus system and sensor quality.

Problem is that a lens for above Canon such as Sigma 70-300mm F/4-5.6 DG Macro (for Canon AF Digital SLR) Lens (Macro Lens) would autofocus, but Sigma 70-300mm F/4-5.6 DG Macro (for Nikon Digital SLR) Lens (Macro Lens) for Nikon in the above body would not focus as D5100 does not have focus motor in built. Both these lenses are for about $170 at the moment. I have few questions here:

  1. If I do buy the above Nikon lens, will it also work in future on Nikon D7000?
  2. In case of D7000, will this lens start auto-focusing automatically?
  3. What do I loose in future on say, D7000 by using this lens instead of an AF-S lens for this configuration? (Which is double the cost of this lens)

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