Tough call on this one as the 'reach' of the crop sensor on the 400D will be very useful for polo and both cameras will be fine for outdoor showjumping but if you plan to do any indoor events the 5D will be your camera of choice for the high ISO capability (in saying that, I have no idea what it is like on the 400D but I presume nowhere near as good) as the lighting used in indoor arenas is usually very poor.
I know it probably sounded a daft question but I keep hearing that the 5D is not good for sports, I know the 400D works as I have got some excellent shots with it
Tobers
Sounds like you need to make a note of what the requirements are that you need, and then choose the camera accordingly, rather than choosing on comparisons that are not relevant to your situation.
I agree 1DMK3Full frame and for sports you want a D3 actually![]()