I am a Nikon guy so I am not sure I am the one to answer. I think rather than Consider the 50D or the D90 you should consider the whole system. Once you buy into one company it often becomes very expensive to change. Both companies produce very good products and I chose Nikon over Canon for reasons that were individual to me and may not apply to you. My suggestion is keep on researching until you feel more comfortable. Once you have almost decided go to the local camera store and play with each model and buy the one that fits best into your hands.

Until then here is a site that might help you

Nikon D90 D80 D300-Canon 50D Compared