They are proprietary PCIe SSDs made for Apple with either SanDisk or Samsung memory chips and a proprietary connector. They're used in all the latest Macs that can be configured with PCIe SSD. There is nothing you can compare them to on the retail market.
No, the drives do not use a proprietary connector. They use a standard M.2 PCIe interface that you can find on all new PC motherboards.
The Samsung XP941 featured in the MacBook Pros and many PC laptops is a popular upgrade for PC users, although you can't find it at retail (it is available new online in PC OEM packaging). The Sandisk is less popular because it is based on older, slower technology and uses 2-lane (X2) PCIe rather than 4-lane (X4) PCIe. Ideally, the new iMac would include a 4-lane PCIe M.2 PCIe slot, as found on the latest PC motherboards, so we could upgrade to a newer, faster SSD.
For the non-techies, the storage hierarchy is as follows:
(1) M.2 PCIe 3.0 X4 (up to 32Gbps theoretical or 3.2GBps real world max)
(2) M.2 PCIe 2.0 X2 (up to 10Gbps theoretical or 1.0GBps real world max)
(2) SATA Express (up to 10Gbps theoretical or 1.0GBps real world max)
(3) SATA 3 (up to 6Gbps theoretical or 600MBps real world max)
(3) M.2 SATA 3 up to 6Gbps theoretical or 600MBps real world max)
(4) M.2 PCIe 2.0 X1 (up to 5Gbps theoretical or 500-550MBps real world max)
The Sandisk 256GB SSD in the iMac 5K is an older M.2 PCIe 2.0 X2 (X2 = 2 lane) device. The theoretical max for that interface is 1Gbps, although the Sandisk drive uses older technology so its actual throughput is 600-700Mbps.
You must have
both a motherboard and a SSD that supports the same interface to achieve a given level of performance. If you stick a M.2 PCIe 2.0 X1 SSD into a M.2 PCIe 3.0 X3 slot, then you will get a max of 500-550MBps, not 3.2GBps. Similarly, if your motherboard only has a M.2 PCIe 2.0 X2 slot, then you will never achieve more than ~1.0GBps no matter what drive you use.
Samsung just released ithe successor to the XP941 PCIe SSD found in the newest MacBooks. This device, called the SM951, is a M.2 PCIe 3.0 X4 SSD with NVMe that delivers 1600MBps throughput (assuming a motherboard with a a M.2 PCIe 3.0 X4 slot) with much lower latency, or nearly 2.5X the real world performance of the 256GB SSD in the new iMac.