It kinda sounds like you made up your mind already if you have the 5DIII on pre-order. Just keep your pre-order position. If you don't then I think you would beat yourself up about it.
Do you really need a backup body?? Would a 1.4x teleconverter work for you instead? It would be a cheaper option of getting the extra reach.
I really wanted the kit and hoped untill the last minutes of the announcement that it would be in the $3600-$3800 range. It ended up to be more expensive than that and my available funds only enabled me to order the camera only.
I order within the first 2 hours of the announcement and you are damn right, I would beat myself up if I canceled my preorder to order the kit and only receive the kit in several months from now.
The back-up body is something that I need because I am a "maniac". The idea of having dust on my sensor is just killing me (as well as dead pixels, and tiny microscopique dings in my windshield). It just drives me crazy (I am probably crazy). Several times, I prefered not changing a lens and miss the picture that I wanted to take because it was too dusty or windy with sand. Having 2 weather sealed bodies, however heavy it can be, it is just peace of mind for me.