I'd chose the FS700 for that. It is more flexible, more versatile, does more things, adapts to more lenses, even has AF for run & gun if you need it.
Now my advise to narrative filmmakers / music video producers / and many others is different, and it comes down mostly to the image and price as there are quite large differences between the two in those regards.
The main issue about the FS100 and FS700 is their highlights, quite digital looking and I think the BMCC has a better image in day light, more cinematic, finer noise grain.
But the FS100 (and I love mine) has great low light, so does the FS700 to a slightly lesser extent. Good dynamic range, but 8bit colour and AVCHD.
240fps is beautiful, I like it for slow-mo.
They compliment each other, if you can afford the expensive gear - $8000 for an FS700 it isn't too much to stretch to another $3000 for the BMCC.
If you can only get one, get the BMCC and spend the remaining money you were going to spend on the FS700 on lenses.
