My 14 year old son is a serious wildlife photographer. He would like to upgrade his older Nikon setup to a newer setup. He is looking at a Nikon D500 with a nikkor 200-500mm lens and he is also looking at a Olympus OMD E-M1 mark 3 with a Olympus 100-400 mm. Can anyone give me advice to cross reference my sons research? Me wants the Nikon setup but I keep hearing that mirrorless is better than DSLR. Any advice would be greatly appreciated. Thanks in advance.