Good comments already.
Insofar as sports shooting, however, Rueben makes a valid point around fps speed - which is much lower in the entry-level than in the mid-range space. The kit lenses aren't the fastest around, and so I wouldn't be surprised should you find some discontent if you are focused on shooting high speed sports photography with an entry level zoom lens.
With regards to a camera that lasts -- I can assure you that your first DLSR body will outlast your desire to upgrade by a very wide margin
The lens selection is essentially equal between Nikon and Canon - however, because the D3100 doesn't have the AF screw drive, you will be limited only to their newer lenses that have the focus motor built into the lenses.
Thinking which will be best for you, I will agree with Jeremy's comment that the camera which feels best in your hand is what you will use the most - pure and simple.