Is there enough of a difference between the 500 and 600mm lenses to justify having both (Severe case of lens lust aside)? The 500 has come and I am loving it. You can still do the monopod, the 1.7 tele extender and walk quite a ways with it even with the battery pack on it. The 600 is much heavier and I am seeing that as primarily a car lens to get small birdies. Is there enough of a difference between the two to justify having both. I sold all my old long glass to buy the 500 and will probably clean house further to get the 600 so it is not totally a new investment.