A) Crucial M550 256 GB NAND Type Micron 20nm 64Gbit sync MLC
B) Crucial MX100 256 GB NAND Type Micron 16nm 128Gbit sync MLC
Both are same price.
What is the best choice.
Didn't this get covered in your other thread?
https://forums.macrumors.com/threads/1840123/
The reason people in that thread recommend the MX100, is because it's usually a little cheaper than the M550 and still a good performer.
Perhaps the MX100 might have lower power consumption (not sure if there is much difference), but for performance the M550 is better (depending on which benchmark and which reviews we are looking at). From my understanding, the performance in the 512GB size is closer between the two, but if the 256GB size is the same price, just get the M550.
By the way, 64Gbit NAND chips in the M550 can be considered an advantage as more (smaller) chips for the same size SSD, can mean more parallel data paths which can help performance.
This is mentioned in the anandtech MX100 review.
http://www.anandtech.com/show/8066/crucial-mx100-256gb-512gb-review/
Due to the use of 128Gbit NAND, the write performance at smaller capacities takes a hit.
In theory the 20nm NAND in the M550 might also have better endurance, but in practice, the NAND in most consumer SSDs should last the life of the machine they are installed into.
Soon the Crucial MX200 will be available and we can compare the options again, but right now if the price is the same the M550 would appear to be a better choice.