I like the idea of a tripod, but I think this will be too heave given the luggage limitations.
A good tripod (Gitzo 3 series for instance) and head (RRS or Arcaswiss for instance) weight around 2.5 kg. How heavy is your bag itself?
I'm always confused when folks talk about the cropped sensor making the lens "longer". Maybe I don't understand the concept very well, but it seems if you have a 5D and crop post-production you end up with effectively the same number of pixels in your image as you would if you used a 50D. Plus, you lose the capability to take a really stunning 13MP shot with the 50D.
This has been written about a lot. What matters in the end is pixel density. A 50D packs 15 megapixels in an area that is 2.5 times smaller than that of a 5D's sensor. It means that the 5D only has about 5 megapixels in the central part of its sensor corresponding to the area of the 50D's sensor.
With a given lens, say your 300 mm f4.0, if you crop the central part of a 5D's frame so as to fous on a distant subject, say a lion trying to eat a giraffe, you will end up with a 5MP image with the 5D and a 15MP image with the 50D, same crop, same lens.
This explain the so-called zoom factor that you enoy with an APS body.
You can of course believe that the pixels of the 5D have a higher quality that those of the 50D, but:
- this is mostly not true due to the fact that the 50D's sensor is 2.5 years newer technology,
- there is no way slightly better 5D pixels could compensate for 5 times more pixels in the 50D (15MP vs 5MP)
So de facto, you can really consider that the actual focal lenght of your 300mm on the 5D becomes a 450mm on the 50D. Ask people shooting birds... most of them use APS bodies.
The day Nikon releases a 15MP D400 I will buy it in a heart beat as a backup for the D3x for those assignements where I need to go longer.
So globally my view remains that, considering your walking scenario, you will probably have a better experience with a 5D, 50D, 300mm f4.0 and a tripod that you would with 2x5D, 400mm f2.8 and monopod.
I am not a safari expert though, but this is what I would do in such circumpstances. Andy Biggs and others should be able to help you more with this.