I think there is a lot of misunderstanding about SATA Express. In the new SATA 3.2 standard that was officially adopted last year the new SATA Express interface/connector was actually designed to be very flexible. It can support standard SATA 3 6Gb/s ssd's and PCIe ssd's. You can connect a standard SATA 3 6Gb/s ssd with a standard SATA cable and it will work. That is as far as it goes for now. It does not look like we will be seeing much in the way of actual SATA Express ssd's. There has been speculation and unsubstantiated rumors that one or two SATA Express ssd's might be available before the end of the year but I have not been able to confirm it.
USAFRet posted a link to a really great article over at AnandTech which explains SATA Express. Everyone should read it for a better understanding of the new header/connector. Currently there are no actual SATA Express ssd's. Temporarily just consider the new SATA Express header/connector as another SATA connector for SATA 3 6Gb/s ssd's.
Although motherboard manufacturers have included SATA Express headers/connectors on new Z97 and X99 motherboards, that is not their actual preference. Both motherboard and ssd manufacturers have a preference for the new M.2 NGFF ssd's and for a very good reason.
For the past several years sales of desktp pc's have been declining while sales of mobile pc's have been increasing Last year mobile pc's outsold desktop pc's. The same thing is happening this year. The new M.2 ssd's were specifically designed for mobile computing. The M.2 ssd's are also suitable for desktop pc.'s. It is a situation where it was more economical to go with M.2 ssd's instead of SATA Express ssd's.
There is quite a bit of flexability with the new M.2 headers/connectors. There are several variations:
1. M.2 PCIe 3.0 x 4
2. M.2 PCIe 2.0 x4
3. M.2 PCIe 2.0 x2
4. SATA 3 6Gb/s
5. Various combinations of PCIe and SATA 3 6gB/S
The vast majority of M.2 headers/connectors on motherboards are M.2 PCIe 2.0 x2. Motherboard companies have also managed to cause quite a bit of confusion. Most of the specifications mention M.2 but do not identify which variation. ASRock is the one exception. The company did a good job of identifying the M.2 variations in their specifications.
There are other variations of PCIe ssd's. They can be mounted on a pcb board which is inserted into a PCIe slot on a motherboard. The same ssd can also be enclosed in a shroud. The ones I saw looked like half height, half length graphic cards. They can also be mounted inside a standard 2.5 inch ssd enclosure mounted in a drive bay and connected to the motherboard with a cable.
Currently the world's fastest consumer ssd is the Samsung XP941 M.2 PCIe 2.0 x4 ssd. Samsung has been mass producing and distributing the XP941 for almost 18 months now. We haven't heard very much about it because it is OEM only. Samsung has lucrative contracts to supply the ssd to computer companies such as Dell, Lenovo, Apple, Acers and others. Samsung never released a retail version for consumers. It wasn't until earlier this year that a few select vendors started selling the XP941 as a replacement ssd.
Samsung is working on their new SM951 M.2 PCIe 3.0 x4 ssd. In addition to the upgrade to PCIe 3.0 x4, the ssd will have some other new features. Among the new features is NVM Express (NVMe) which will be replacing the old ACHI mode. If all goes well mass productin and distribution to OEM partners will start in about 6 months. It is not known whether a retail version will be available for consumers. Two days ago ASRock released their brand new X99 Extreme11 motherboard. The board is equipped with two M.2 PCIe 3.0 x4 headers/connectors. The bad news is the board is equipped with so many extra features it costs $649.99 USD.
I maintain the ssd database listed in a sticky at the very top of this forum section. Here is the link:
http://www.johnnylucky.org/data-storage/ssd-database.html
The database includes seperate PCIe and M.2 categories for each brand plus links to published technical reviews.