Pay attention to the lenses that are available for the body. That's the best way to decide which system to go for. After that, look at the bodies available for the system and get the best one you can afford. But, the lenses are where most of the money goes anyways, so if you have to scrimp, do so on the body, not on the lenses.
Quality lenses will last decades, you'll probably replace the body at least once during that time.