Neither!
Both!
No but seriously, they are not comparable.
The a6500 is built for both photography and video applications.
The a6500 has a 24MP APS-C (23.5 x 15.6 mm) sensor. The sensor’s overall size 366.6 square millimeters.
The Sony a7S II is built specifically for video applications, has a full frame sensor that is over twice as large (35.8 x 23.9 mm), with half the amount of pixels (12MP). The sensor’s overall size is 855.62 square millimeters. That is over twice the size of the a6500’s sensor… the pixels are much larger than the a6500’s leading to exceptional low light performance.
They both do 4K video, they both have the processing power to handle high frames rates, but one is a purpose built tool and the other isn’t.
If you’re looking for the highest quality, almost cinematic quality, then the a7S II is your best bet. It’ll give you that indie movie feel that we both know you want straight out of the box. You also get your choice of full frame E-mount lenses. The software is also different with the a7S II offering more supported resolutions.
The a6500 though, has multiple jobs to do and while it can use both full frame and APS-C lenses, the focal length is different, which ultimately can change the nature of the lens. When your 35mm prime becomes a 47mm prime, things look a little off, and that can annoy people. But the a6500 lets you take fantastic photos as well as great video.
To boil it down, the a6500 is for a photographers who also do some video work.
The a7S series is exclusively for video applications (the photos suck).
Don’t forget that the a6500 costs significantly less than the a7s series. You could buy both an a6300 and a6500 on sale for about what you’d pay for the a7s. That’s meaningful for anyone who needs a backup camera or has a more complex shoot