Portra 400 would be fine at box speed with a strobe.
But since you WANT light falloff in the background, I would thunk Ektar would be better because it falls off quicker in lower light, but that will end up a bit bluer, try a test shot before the actual shoot, maybe throw a warming filter on to compensate for the blue cast of ektar or use a warm tone gel on the back light strobe?
It's really individual and you haven't shown examples of your digi work to show what you want to accomplish.
This is ektar with a strobe outside... It's NOT a good example, but you can see how the dark stone in the foreground even just shade falls off quickly
If you shot E-6 I would suggest Provia100f as your best option...
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