For wildlife, yes, the 100mm is absolutely worth the extra $1600. It might help to know that the 600mm gives you more glass for your dollar; it costs 9% less per square millimeter of aperture:



$5800 500mm f/4, 125mm aperture, 12272 sq.mm, $0.47/sq.mm



$7600 600mm f/4, 150mm aperture, 17671 sq. mm, $0.43/sq.mm


The really difficult question is the weight. 3.3 pounds makes a big difference when you're backpacking and most other situations as well. Higher weight and longer focal lengths also require higher standards from your tripod (which may take yet more weight to improve), technique, and (sometimes) atmospheric conditions (e.g. big animals from long distances).