Canon SLR Cameras

I’m not that much of a camera whiz, but I’ve outgrown my point and shoot powershot camera. I pretty much mastered it. Took great action shots with what limited options I had to work with, but its really time to step it up!
I need something I can take action shots of quads and portrait/action shots of horses.
I’ve really been torn between these two cameras. The Xsi and the 50D. I would really like to hear everyone opinions on these two styles of cameras.

I don’t have any experience about the actual camera or lenses but I want a fisheye lens. I’ve been recommended some, for example the tokina 10-17mm, sigma 10mm or peleng 8mm… but also can anyone recommend where to buy one from online? Can’t believe how expensive they are!