Well, I'm a Canon shooter, so I've gotta argue the other side.
Since you put wildlife (at least I'm assuming that's what you meant by animals, not your pet cat
) and sports on the list, I'd actually like to point out that a camera body is a wonderful thing, but it's digital equipment. You're likely to want to upgrade in three to five years' time.
Your permanent purchase is going to be the glass: your lens collection. And when you choose a brand, you're locked into a mount system. Better to figure out if the lens lineup suits you BEFORE you've sunk a few thousand dollars into a system.
Sports and wildlife require telephotos and supertelephotos. And sports in particular demands FAST telephotos/supertelephotos (well, depending on the sport).
And Canon's lens lineup, while not necessarily better than Nikon's, sometimes has more offerings over a larger variety of prices, once you get above the consumer level. If you're planning on staying at the consumer level (<$500 per lens), then there's probably not much to choose among either brand, so go with the feel of the camera body. But if you think you might end up wanting prosumer or pro glass, then glance through the lens lineups and see what your dream lenses are going to be--consider what you might be able to afford in ten or twenty years.
Just as an example (and since I have the Canon bias, this will be in Canon's favor. You'll find other examples in Nikon's), say you want a prosumer/pro-level telephoto zoom that goes to 200mm and that's faster than the wimpy f/5.6 of consumer lenses.
On the Nikon side, you've got two choices:
($900) AF Zoom-NIKKOR 80-200mm f/2.8D ED
($1600) AF-S VR Zoom-NIKKOR 70-200mm f/2.8G IF-ED
On the Canon side, you've got four:
($600) EF 70-200 f/4L USM
($1000) EF 70-200 f/4L IS USM
($1000) EF 70-200 f/2.8L USM
($1600) EF 70-200 f/2.8L IS USM
On the Canon side, you can choose if you can afford either stabilization or f/2.8 or both or none. Also, for that "rock-bottom" $600, you could get a much smaller and lighter 200mm f/2.8 prime. While Nikon's 200mm prime is f/2 and $4000.
Canon has three tilt-shift lenses. Nikon only has one. Canon has an 85mm f/1.2 lens; Nikon doesn't. And all the Canon EF lenses (well, ok, except for those tilt-shifts, but they're TS-E, not EF
autofocus on the bottom-of-the-line XTi. Do any these factors matter to you? Maybe not. But if you were into architectural photography, or professional portraiture, or shooting something fast-moving with an inexpensive dSLR body, then maybe they would.
Look at the lens lineups. Learn what the focal lengths and maximum apertures mean, in terms of what kind of tools they could be to you. Look at the specs and figure out the size and weight and cost of the lenses. And determine which lineup is going to be better for you. Then, your brand is chosen, and you can figure out which body you want or can afford.