My humble 2 cent is to hold the camera... play around with it and see which you feel most comfortable with.
Or just get what "brand" your friend is using to leech lens.

In short...

Canon:
- if don't do post processing, jpeg colors are more muted. Advantage : skin color
- EOS system's motor is in the lens so all modern Canon lens can be used. (FD lens from the 1970-1980's cannot use)

Nikon:
- if don't do post processing, jpeg color more vivid. Advantage: Landscape
- Nikon's lens system a bit more complicated http://bythom.com/lensacronyms.htm
- D5100 video can autofocus (well.. almost)
As above. It's purely preference.
This is more dependent on yourself as the shooter. Cheap lens no good, good lens not cheap. All brands also have one or another. There's also old manual lens that's cheap as dirt but good though you have to pusing pusing to focus.