I own a Gh5 with meta-bones speed adapter. I am not a fan of Sony, but definitely understand why some people like them. I've found the GH5 footage has more neutral colors than any sony camera I've used. Sony's have always had this look to them, the color shifts are very noticeable. I find that when matching RED and GH5 footage it is much easier than many other cameras.
So the first thing I like about the Gh5 is that it has more neutral colors. Especially the a7sii. Because the Gh5 has a bigger bit-depth. pretty sure the Sony a7sii can only do 4:2:0 8 Bit where the Gh5 does 4:2:2 10 Bit. Basically the Gh5 stores more information when recording. This makes you able to shoot in a flat profile like v-log so in post you can utilize the entire dynamic range from the camera. This is very simplified, but watch a video on youtube about bit rates and log if interested.
Also it's pretty sick that you can shoot 4k 60p on the Gh5, which would use the lower bit rate. 4:2:0 8-bit. Even when using this bit depth it is more neutral than the Sony. Using Cin-D color profile will give you a very neutral image on the Gh5. But you can get an external recorder, which is basically a monitor that has a computer in it that helps boost your camera. If you buy an Atomos recorder for the gh5 it can shoot 4k 60p at 4:2:2 10 bit. This is a crazy file and honestly is huge. the a7sii is not getting anywhere close technically.
Another part of the gh5 that can be nice is the lenses. If you have a speed adapter, it is making the censor size tec. bigger also giving you the option to use canon lenses. EF lenses are the most popular and abundant lenses in the market. Lenses are more important then your camera, they are where the real creativity comes in when filming. Sony lenses are sooooo expensive, they are really good, but many of the good sony lenses alone would be 2,500 to 3k. You can always get an adapter from sony to EF so it's really not that big of a deal.
The one thing to mention about the a7sii is its low light and that it is a full frame. The full-frame allows you to have more light, so better bokeh and depth of field. It also has super large pixals, which makes it so you can shoot super high ISO's. This is soooo useful and can honestly be a game-changer. I would love to have a sony a7sii in my camera bag just for shooting night stuff. But you can't have everything in the world so you got to make a choice...
Its not about the camera, its what you're pointing it at. Cinema cameras are not good in low light. Even the Arri's kinda sucks in low-light. You need to light your scene, lighting and bending light is what makes a good-looking image. If you boost your ISO to make the image brighter, you are now not using your native ISO and your shadows will suffer, your dynamic range gets crushed.
I think the Gh5 looks really neutral and has extremely competitive codecs that the a7sii does not have. You will have much more room in post to color correct, which would also give you a taste of what is to come with more complex CODECs like Red Raw.
Many film schools get Gh5's because they are very versatile and are far more professional video oriantated. The sony alpha series, are hybrids. The Gh5 is a video camera and you get the waveforms, anamorphic modes, zebras, so many preset buttons. Also the menu is super good. Sony has terrible and confusing menus.
That's my rant. Heavily bias. Sonys are dope don't get me wrong just do your research. There is a lot to cameras and every person needs a different type of tool.
