I echo most of the advice you've had so far. The D5100 has a bit of an edge on the T3i (T4i perhaps not so much), but not enough to rule out the T3i if you prefer the feel. Investing less in the body (entry level or used/refurb prosumer) and more in the glass is a wise move. You'll definitely need more reach than 55mm if you want to do sports photography. You'll also want to upgrade any kit lens down the road.
I highly suggest reading up at Photography on the Net (Canon) and Nikonians (Nikon) prior to making any camera related decisions.