If you're looking to buy a Samsung SSD, I would get the new 850 Pro series that was just released a week ago.
The 850 Pro has a 10-year warranty on it compared to the 3-year warranty for the 840 EVO, and the 5-year warranty for the 840 Pro.
Any SSD with a 10-year warranty is definitely worth buying.
The SanDisk Extreme Pro also has a 10-year warranty.