Your motherboard supports m.2 pcie devices.
My suggestion is to buy a single 1tb 970 evo.
You will get more total capacity, a faster ssd and easier space management.
Just install windows on the drive and you only have one C drive.
Very easy.
If the price seems high, the intel 660P 1 tb m.2 drive is an alternative.
No as fast as the 970 evo, but still much faster than any sata ssd devices.
In the early ssd days, longevity was an issue since a ssd does have a limited number of writes for each nand block.
But, today with 500gb and larger devices, there are so many nand blocks availavle that your ssd will be obsolete long before it wears out.
As to reliability, with no moving parts, a quality ssd is more reliable.
Particularly if you buy intel or samsung who can have better quality control over their own parts.
Do not worry about specs. What the drive does is what counts.
Do not be much swayed by vendor synthetic SSD benchmarks.
They are done with apps that push the SSD to it's maximum using queue lengths of 30 or so.
Most desktop users will do one or two things at a time, so they will see queue lengths of one or two.
What really counts is the response times, particularly for small random I/O. That is what the os does mostly.
For that, the response times of current SSD's are remarkably similar. And quick. They will be 50X faster than a hard drive.
In sequential operations, they will be 2x faster than a hard drive, perhaps 3x if you have a sata3 interface.
6X with a pcie interface.
Larger SSD's are preferable. They have more nand chips that can be accessed in parallel. Sort of an internal raid-0 if you will.
Also, a SSD will slow down as it approaches full. That is because it will have a harder time finding free nand blocks
to do an update without a read/write operation.
Larger ssd devices have more endurance.