You'll find 256GB SSDs sprawled across a wide swath of prices. Some of the new SM2258-controlled models armed with Toshiba 19nm TLC start as low as $70. The top-performing drive is SanDisk's Extreme Pro, and Newegg has that one at $115. You'll get your best value from Samsung's 850 EVO 250GB, which sells for $100.
Currently, Plextor's M6V is available only through Amazon in the U.S. The company has its drive listed at an MSRP of $100—the same price as Samsung's 850 EVO. The M6V has been available in Europe for a month or so longer. More European vendors have the M6V in stock, and EU pricing has the two competitors better-separated (with the M6V available for less). Here in the States, we should see Plextor's offering become less expensive in order to compete.
It's often difficult for Plextor SSDs to go head-to-head based on pricing. Its drives come from Lite-On, and the Plextor name is licensed. A percent of the sale goes to Plextor, adding to what you pay. To help offset the added weight, Plextor chose to roll its M6V out with 15nm Toshiba MLC. It's not clear if Toshiba is charging less for the new lithography, but we know it costs less to manufacture 15nm flash than the 19nm NAND, since Toshiba can harvest more dies per wafer. Because 15nm is still new, we don't know how yields are doing. If the price is the same now for 15nm and 19nm, you can bet it'll change over time, with 15nm emerging less expensive.
Performance-wise, Plextor's M6V closes the gap with Samsung's 850 EVO, but doesn't manage to overtake it. The two compete viciously, offering similar performance and software features. The largest distinctions between them are probably encryption support (Plextor's M6V doesn't support it) and warranty (Samsung gives you two more years than Plextor). To be fair, I haven't yet heard any low-cost SSD buyers arguing for FDE. The technology is still fairly new, and most home users simply don't care about it. Warranty terms are much higher-profile. Longer warranties instill confidence in a product, and the M6V falls short there.
Let's cut to the chase: does the M6V top Samsung's 850 EVO? In some ways, yes, in others, no. It's a solid product from Plextor, but I don't see a reason to buy it over the 250GB 850 EVO at a similar price. If you were bothered by the 840 EVO issues and want to stay away from Samsung for a while, the M6V is next in line with Crucial's MX200. Both are better than the Toshiba TLC-based drives that share their air.
At Computex, Plextor announced several software features designed to reduce the amount of data held on the SSD. We hoped it'd release PlexCompressor for the entire portfolio. But right now, that software is only available for premium models. PlexCompressor for the M6V could certainly improve the drive's value, and it may find its way to Plextor's lower-end models. But I think the company, along with its competition, is more worried about compatibility with Windows 10 right now.