Consider your workflow...do you have the capacities to deal with 4K ?
Do you need the extra power ? (I am shooting short fashion films for websites, and HD is still the gold standard here, many clients could not even handle bigger files etc. at present time.)
Consider your budget - you will need more money in the beginning, to get a kit ready for shooting:
high end SSDs (although the price/value level is better in SSDs than in high end SDs), a reasonable external power solution, most probably a cage of some kind.
I was in the same situation (having a fully equipped 5DmIII Kit with several nice L lenses) but nevertheless decided to go the "pocket way" with the metabones adapter and the Sigma 18-35 for Nikon. I do not have any Nikon bodies, but the lens re-sale values are ok.
Plus, consider the extras, which the Metabones Speedbooster for BMPCC will add: focal reduction, f-stop improvement.
If you do not need much more, I would go with the BMPCC kit - it is a unique solution with the Booster and the Sigma and still very managable, even for stealth filming or holidays