despite other differences, please correct me if I am wrong after looking for sensor sizes of both cameras:
bmpcc 4k + speedbooster 0.64x = crop factor 1.21x (with +1 fstop extra)
bmpcc 6k = crop factor 1.55x (can't use a focal reducer)
I know there is other considerations in favor of 6k like resolution, native mount without adapters, etc, but seems that DR and SNR are similar, without no benefits of larger sensor, at least on initial paper.