With the lenses you are bringing, the 5D may be the better choice. Here are my two main reasons. Based on a quick handling of the camera and my use of the 20D, the 5D felt very similar to the 20D. You probably do not want to be using a completely unfamilar camera for a trip like this. The second reason is that you have some focal length duplication with the 100-400 and the 300 and 1.4x. Using a second full frame body will give a lot more flexibility compositionally. Use the 300 and 1.4x converter with the 20D and the 100-400 with the 5D and I think you will get the best of both lenses. This is more a personal preference, as if all other things are equal, flexibility tends to be more of a priority for me.
Other pros and cons: The 5D is not the 20D, so there will be some learning involved. Larger file sizes for the 5D, so memory may be an issue. No cropping factor for the 5D, so your wide angle zoom will be more useful than with another 20D, as a wide angle. Larger prints from the 5D, if that is a priority. You may like the 5D so much that you run out and buy one after the trip (Hey, we all know that Canon needs the $3,200 far more that you do ).
I hope this helps.