It depends on what you want. I've shot two short films, one with a Blackmagic 4K, and one with a Sony A7s. I also use a Sony FS5 and FS7 for my day job. Working with both cameras was great for each film. It depends on your needs:
If you need low-light capabilities, go with the Sony. Blackmagic has terrible low-light.
If you need slo-mo, go with Sony.
If you want to save money on media, go with Sony. You can record with SD cards, whereas on the Blackmagic you've got to buy SSDs. Around $130 for one right now, versus the memory cards on the sony will only run you back around $40.
If you want a better color grading experience, go with the Blackmagic. My experience with sony color-grading has been much more painful. You can do it, but it will be much harder, just because of the weird skin tones that come from these cameras. Sony comes with 8-bit color, Blackmagic comes with 10-bit or 12-bit depending on ProRes or RAW, respectively.
If you want to have more easily editable files right out of camera, go with Blackmagic. It shoots in ProRes straight inside the box, which is very easy on NLEs. You can do RAW as well, but the added file size and extra transcoding may not be worth the extra dynamic range. It wasn't for me.
As far as resolution is concerned, the 2.5k you get from the Blackmagic will look better than the 4k you get from the Sony a6300. That is an opinion, but I just really love the image from the BM. Also, the bitrate you get from the Blackmagic will be higher, even in the lower resolution. And as far as the 4k Blackmagic is concerned, you get better dynamic range from the 2.5k model anyway.
I hope these tips help! My personal choice is the Blackmagic. You get less features, but the images you get out of the box really sell it for me.