Complex question, let's break it down into parts.
Performance. You don't need raid 0 because SSD's are so fast. It will make no noticeable difference. Also, the difference in speed between the 850 pro and 850 EVO is insignificant, go with the EVO. And the larger a SSD is, the faster it is, up to 500GB.
Durability. Do not go with raid 0 for two reasons. One is that you have two units so you just doubled your chance of failure. Also, SSD's are more reliable the larger they are, for you have more memory to write to.
So, go for a Samsung 850 EVO 250GB. The 850 has stacked memory which is what you want for the individual memory cells are much larger and more reliable then the small cells of the 840 and all the other latest SSD's by Crucial etc.