To try and clear up this confusion. All Canon bodies have a built in auto focus motor. That means that they can auto focus with any auto focus compatible lens. If a lens has its own built in motor, it will use that instead as it is generally faster and quieter.
Nikon's lowest end bodies do not have built in auto focus motors. Any body without a top LCD screen will not have its own AF motor, including the D3100, D3200 and D5100. This means that to have auto focus you need to use a lens with its own built in motor. Nikon offer such a huge range of these that this is of no real concern these days. Third parties, such as Sigma, Tamron and Tokina generally now fit auto focus motors into their lenses as standard as well as it generally offers faster focusing.
Erm, basically, what I'm saying is, I wouldn't worry too much about it either way.
Thanks for this - doesn't help me decide at all!