Thanks guys,
Speed isn't really an issue as long as I can keep timing down to with a second or two (timing as in when events happen or get triggered). and I would probably want to keep it Simple and it would be two way communications.
So if I go with Serial to communicate with the 2 MPU's I can still use one for I2C for things like ADC's or Temp sensing?
That is why the question arose to begin with. If I am going to be using I2C anyways, why not communicate with each other as well, but - Only one Arduino will be need the I2C. So I guess now the question becomes is it harder to use both Serial for the MPU's to communicate and one of them I2C or just do all I2C?