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.

Product Description
The NB-4L is a rechargeable Lithium-ion battery pack for Canon PowerShot models SD200 and SD300. It’s made to withstand approximately 300 recharges. When shooting with the LCD monitor off, you can expect approximately 400 images when fully charged, or 140 images when shooting with the LCD monitor on under normal environmental and shooting conditions. Similarly, the battery offers about 180 minutes of image playback time…. More >>
Canon NB-4L Battery Pack for the SD400, SD630, SD600, SD750, SD1000 & TX1 Digital Cameras