The A7S III and FX3 are basically the same camera with a different case and additional cost. The A1 is a great allrounder but the A7S III can still beat it as a video first camera. (https://wolfcrow.com/sony-alpha-1-vs-sony-a7s-iii/) The FX6 is really the odd one out here and you will need to compare form factors for your use case. To me the race is between the cheaper A7S III with a Mirrorless body versus the cinema camera form factor FX6. If price is a question then A7S III for sure. Also, you can use the A7S III on a gimbal - the FX6 probably not.
I think the biggest thing I think about is the electronic variable ND filters of the FX 6. It seems so perfect. I also already have a few Sony mirrorless cameras for photography I'm not sure I want that same body. But then it's also basically half the cost. THIS IS SOOO HARD
What were your reasons for grabbing that one over another? Another thing I'm thinking is that right now it's on sale. I want the variable NDs of the cinema body but it's $6k. But I also don't want to buy it and feel like I wasted my money. In the same way, I don't want to NOT get it and feel like I should've gotten the more expensive one.
It's not "just" electronic NDs. These are internal, meaning no need for multiple expensive ND filters. And then they're intelligent with an "Auto" ND mode that can maintain proper exposure settings with 0 shift in color. It's regarded as the greatest electronic ND system of any modern camera and that's no easy feat considering all of the other awesome cameras from people like Canon. There's the body style and form factor, the audio quality, etc. But, you can get a similar image from an A7S III. This is why it's a tough choice.
The FX 3 and FX 6 are in the official Sony cinema line. Plus I own all Sony lenses, I'm don't want to invest in Canon lenses.
Yeh that's what I meant. There's quite a bit of other stuff to it too. At the end of the day you know your requirements the best. Fo me personally the FX6 is for special stuff so will rent it. Much easier to own the A7sIII since I already have an A7III. If the eND, Audio and form factor justifies the cost then just go for it. In my country renting an A7sIII costs say 70% the cost of renting an FX6 for now., And the A7sIII costs say 100 days of Rent so If I have 100 days of paid work over the lifetime of the camera then it is as good as paid off. Do your own calculations and you'll know which one to get.