I'll assume you're asking about SLR systems, i.e. Nikon vs Canon vs Minolta vs Pentax.
I'd recommend going with Canon as the system of choice. Canon "bit the bullet" big time when they dumped the old lens mount and went with a whole new one, but it has paid off.
Most sports photographers are using Canon now, because the lens design means that the focus motor is in the lens, and thus is customn designed specifically for that lens, whereas all the others use the one motor in the body to drive all lenses. This gives a huge advantage in getting the optimum performance out of an autofocus system.
Another advantage of the large Canon lens mount - (have a look at the size of the lens opening of a Canon vs a Nikon body) is that you can make a much faster lens. Canon currently has the only f1.0 lens, and a raft of excellent fast lenses.
I've used Canon autofocus since they first came out, and have never had any trouble with them, and that includes hauling them around the world.
Susan uses a Rebel G, which is the lightest thing you can imagine, and I use the 1N. Both superb cameras. They have both ben cleaned once, after Africa and a lot of sand and high winds in Namibia, and both were fine, albeit with a little sand inside. Changing lenses in a sand storm isn't the best idea
If you want really durable, get the pro models, a 1N or Nikon F5. They both have better sealing and durability than all the other models. They cost more but are worth it if you want tough. Don't bother with the optional booster motor, the standard built in is fine and much lighter. I have the booster and never use it.
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