Have you looked at the lenses? What do you plan to shoot and what lenses do you want? The camera bodies are going to make the decision tough, because they're all good these days, and often quite equitable so that you're looking at small feature differences.
Remember that when you buy a camera body, you're more or less locking yourself into a mount system. So looking at the lens lineups and what the lenses cost is another way of figuring out which system may be a better fit for you.
A camera body is now computer equipment. Like most computer equipment, you're likely to feel the need to upgrade every 3-5 years. The glass is going to be a more permanent purchase and can last you decades.
Learn about lenses; they're the other half of your decision, and will probably eventually cost you two to three times what your camera body will.
One more thing--go play with them in a store and evaluate handling ergonomics and menu logic first hand. That may also help weight one camera over another.