Allan,
I think dust on the sensor is inevitable, regardless of whether you use 2 camera/lens pairs or change lenses.
Zoom lenses, such as the 70mm-300mm must allow air in and out. Otherwise, when you slide the lens body up and down to zoom, the lens would snap shut because of the vacuum.
One thing I have found useful in the dust arena is switching the camera off between groups of shots; not between every shot but rather on a "now & then" basis. This way, the camera sensor cleaning routine that the camera goes through when switching off should at least prevent a build up of dust.
Incidentally, I too am using the Canon 70mm-300mm but the black version. I read a couple of recent reviews of this lens and they each said this lens was very close to the quality of the white version and probably not worth the additional '1' on the leading edge of the price! I have to say I am very happy with it but I have discovered one drawback; if I try to use the full 300mm, I will see softening on the extremes. Aircraft noses are a good example of this. The answer is simple; stick to a max of around 270/280mm.
Kelvin