Short Answer: This depends on how likely Magento are to make updates to Blank/Luma.
In the past it has been advised you DO NOT use the Luma theme as a parent, as this is Magento's demo theme they are more likely to update this without warning.
Alan Kent (Magento chief architect)
The Luma theme is not designed to be inherited from - it is purely developed as a demo store for people to explore and play with
Taken from https://github.com/magento/magento2/issues/2547
I have just noticed that approx 1 year after making that comment Alan said this:
I checked with the PM's more recently, and they said aim is now to allow people to inherit from Luma. We might need to clean up a few things over time to make this "nicer
Source - https://github.com/magento/magento2/issues/3220#issuecomment-177296871
So it sounds like they are aware people are using Luma as a parent so in my opinion it shouldn't really matter which one you use.
The flip side
On the flip side you can still use Luma as a parent without issue, I have built one project using Luma as a parent and didn't run into any Luma specific issues.
What I did to workaround the 'Magento may update Luma' issue is to clone the Luma theme naming it lumaClone. And then used that as a parent, the downside of this is you have to manually make all the template changes made in patches.
My recommendation is to create your own blank theme (not using Magento's Blank or Luma themes), and use that as a parent. But of course that takes a considerable amount of time to do so not everyone will be able to do so.
But this gives you a better starting point, more flexibility, and more control. With the downside patches will potentially involve more manual work.