Memristors have many advantages over flash, but the areal density advantage is not a large one. My money is on the nonvolatile RAM (memristor) over the large-block-addressed and limited-rewrite Flash, but I also expect that multi-Terabyte disk drives will be around for the forseeable future. A lot depends on whether the HDD manufacturers have built the bit-patterned media plants and the two-digit-Tb drives before SSD eats their bread and breakfast market. They might decide to stop investing in future bigger HDDs because the HDD business doesn't have a future (which would be a strongly self-fulfilling prophecy).
The price of N Terabytes of SSD will always be N times the price of one terabyte (until they can make a 1Tb nonvolatile storage chip, if ever). The price of one disk drive will be £50 plus whatever they can get for it being bigger than cheaper ones. If a 10Tb or a 50Tb drive is ever marketed, it's a fair bet that 5 years later it will be available for £50 of today's money.
Wafer-scale SSD integration might one day put a terminal spanner in the HD works, but wafer-scale integration is something that's been coming for almost as long as nuclear fusion, and like fusion we still don't have it.