I agree at first glance the differences between the 500D and 50D are very similar. I found this comparison on dpreview which sums up the big differences. So when you're trying to get those hummingbird pics, the 50D is going to be your best friend.

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The key differences between the 50D and T1i isn't about image quality rather being more likely to capture the image you want.

The 50D is much faster; 6.3 frames per second over 3.4

Much brighter, larger viewfinder as the 50D uses a Pentaprism rather than Pentamiror

50D offers 2 complete user configuration settings which can be accessed instantly via the mode dial. An example of usage; he's at a motorsports event and 1 configuration wold be for AIservo tracking the cars while the other is for shallow depth of field oneshot cacdids as a famous driver might just be walking by.

The controls to change settings is much easier to change as well as faster.

The 50D focus screen can be changed i.e. to one with a grid for easier alignment

The AF system is much faster and more accurate as well as having 9 Cross type AF points rather than just 1. The 50D centre AF point is a cross and X style for added accuracy.

The 50D AF sensors are also much smaller so you can more specific about the focusing spot.
ie. T1i

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