I recently posted about the benefits of trying to push your image as much as possible in this thread, and I this is a good strategy for you here.
I'd suggest:
Shoot a side-by-side test between the Sony and the BM cameras in increasingly low lighting conditions so you can see how far each is capable of going into low-light conditions
Grade the side-by-side footage as hard as you can, pushing as much contrast, saturation, and pushing and pulling it as hard as you can
Grade each to match the other, seeing what is practical in matching them, and this will also reveal the little nuances of each of them that won't be obvious when you're just grading creatively
This will enable you to understand the relative strengths of each, will push you to learn more about colour grading (over the years I've been able to learn how to push things further and further, which gives me more freedom in post to create whatever looks I want).
The Sigma FP is a great camera, if a little finicky to use and with brutal data-rates for the RAW codecs. I was seriously considering it for some time and the low-light is quite impressive.