This is my first 600mm F4, so I have nothing to compare it to. But I had the Nikon 300mm 2.8 and this feels much lighter. The quality of the images is outstanding and the focus speed as well. You will not be disappointed. I've also been using it with both the 1.4 and the 2.0 teleconverters. I was highly skeptical about the 2.0x but from my last session it is wonderful on the a9ii. Focus speed is fast and the quality of the images is great. The only thing I need now is to be able to record 4k in 120f. I put in an order for the a7siii and I hope there will be a competitor to the Canon R5 in 2021. I don't think Sony has a choice tbh.
I have the Nikon 500F4E ED FL and like to hand-hold so did not even consider the 600FL. Ordered the battery grip and the TC 1.4 which should still be fast AF/great IQ and give me 840mm. I shoot primarily on a very wide river so the birds can be very far away. do not do any video as of yet, but would be nice to know I have the capability. Yeah, seems Canon has the edge with video on the R5 but being a Nikon shooter and now adding the Sony, the last thing I need is to add a 3rd system LOL