Was in exactly the same place only a month ago.
I really like the BMPCC image and the codec is great but don't write off the GH4 either.
It looks too 'video' out of the box but add the speedbooster and fast glass plus some in camera profile setting changes and it's a different beast completely. And a beast with 4K, a great battery, variable FR, and a pretty decent codec.

Thanks a lot for the comments. I do really like the look of the BM footage but am now pretty sold on the GH4 with speedbooster. I've been concerned that it looked a bit too 'video' but found a few videos with the Sigma/speedbooster combo which look the goods.
The 4k, high fr, and battery life just sealed the deal. The 8 bit internal codec is the biggest drawback plus the low light performance, but the BMPCC hardly sounds like a low light warrior either.
Certainly considered the A7S but think it's out of my budget.
Now, tell me I'm stupid for going GH4!
Thanks again.