This question has been asked many times. Here's a copy/paste of a previous response....
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It's all about the fit in your hand. The price of the XSi is rather steep for an entry level, I'd compare it to the Nikon D80. Go to a good store, play with both Canon's, and try out the Nikon D40x and D80 while you are there. What feels more natural to you? The camera should be an extension of your hand, so it needs to be comfortable and somewhat intuitive. It's only a light-tight box with a hole in it. What matters is the skill of the person holding the camera.
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There are a lot of really good threads on this topic (one camera vs. another)
Here are a bunch of replies I've posted, maybe they'll help...
http://www.e90post.com/forums/showpo...8&postcount=26
http://www.e90post.com/forums/showpo...8&postcount=29
and check out this entire thread. It's worth reading at least the first couple pages...
http://www.e90post.com/forums/showthread.php?t=94255
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Remember, the brand is meaningless. Canon or Nikon, both are outstanding. Try them, see which is more natural and comfortable.