SATA Express: stillborn?
When the SATA Express standard was developed, its primary purpose was to connect 2.5″ SSDs in desktops over PCI-Express. The connector looks like two regular SATA connectors next to a third mini-connector. As a result, SATA Express is compatible with Serial ATA. The Intel Z97 and X99 chipsets have the capability to either use the entire contraption as a SATA Express connector (with PCI-Express interface), or the two internal connectors as two individual Serial ATA 600 connectors.
SATA Express has one important disadvantage compared to PCI-Express and M.2 add-in cards: a SATA Express cable can only transport two PCI-Express lanes. When using PCI-Express 2.0, the maximum throughput rate is therefore limited to 1000 MB/s, which is a relatively modest improvement over Serial ATA 600. We don't know whether SATA Express is also suitable for PCI-Express 3.0, although we do know that Intel's current implementation in the Z97 and X99 chipsets is not.
Admittedly, the lower speed is not the only reason as to why most motherboard manufacturers aren't convinced by this standard. SATA Express will by definition only be used in desktop computers, as M.2 is the standard for laptops. Chances are that the various SSD manufacturers won't be releasing a whole lot of SATA Express SSDs for the desktop niche market. ASRock, for one, has already pronounced the standard to be dead before the first SATA Express SSDs have even seen the light of day. ASUS, on the other hand, actually incorporates two SATA Express ports on some of their motherboards. One of these is connected directly to the Intel chipset, and the other to an ASMedia controller that also offers PCI-Express x2 with 2x SATA600 as a fall-back option. That said, an M.2 connector can be found on literally all X99 motherboards that we have tested thus far...
As was mentioned before, the SFF-8639 connector has been developed specifically for servers. Signalling over SFF-8639 cables is a lot more complex, with one of the reasons being the need to include a clock signal. These cables also require a lot more shielding, substantially increasing their price. An advantage of SFF-8639 is that it can use four PCI-Express lanes, rather than two. As a result, a 2.5″ SSD with an SFF-8639 connector can be just as fast as a PCI-Express add-in card.