Performance-wise Intel's SSD 750 400GB and Samsung's SM951 are fairly even. The benchmarks prove that both drives have their strengths. And under normal desktop workloads at low queue depths, I'd go so far as to call them equal. The SSD 750 400GB serves up stronger performance at high queue depths for workstation users. NVMe also allows the SSD 750 to deliver better latency under heavy load, which carries through during SSD recovery phases (one of the most important considerations if you're actually able to stress your storage subsystem).
On the flip side, Samsung's SM951 delivers identical performance at low queue depths and does so while giving you more available capacity for user data. The SM951 512GB is also compatible with a growing number of notebooks. You may not plan to use it in a mobile platform now, but when the next generation of enthusiast-oriented SSDs emerges, at least you have the option of moving it into an aging Ultrabook.
Most enthusiasts pay less attention to the power consumption of storage devices. But Samsung has the advantage in this discipline, for what it's worth. The M.2 form factor is heavily restricted when it comes to power, while PCIe add-in cards can use nearly 5x the power and still be in compliance with the standard's specifications.
There are two areas where you'll see big differences between these two products. The first is price. Samsung's SM951 costs nearly $100 more than Intel's SSD 750 400GB. But the SM951 512GB also gives you more than 100GB of extra capacity, so it's not like you're paying more and not getting anything in return. I suspect the capacity difference will matter more to power users than the price.
We saved the best bit of info for last. Samsung plans to release its SM951 in two versions: the AHCI model we tested today and a future NVMe-based drive that's still on the horizon. I really didn't foresee NVMe having significant compatibility issues, but you still have to take this into consideration. The NVMe-capable SSD 750 works in a wide range of computers, but getting the drive to boot into Windows is another story. If you have a motherboard with an Intel 9-series chipset, you should be safe; board vendors are already updating their firmware to support the NVMe interface. Outside of this generation's core logic, though, your odds decline precipitously. Samsung's AHCI-equipped SM951 SSDs naturally work in many 8-series platforms and even several AMD based motherboards.