Hi evil_maverick,
I'd say It all depends on your budget. SSD is expensive, however, they are worth every penny. The difference between Samsung 840 Evo and 850 pro are in terms of performance and technology. Samsung 850 pro is based on V-NAND technology and have better IOPS, as compared to the Evo models.
The biggest difference that I can find though, between Samsung 850 Pro and Evo is the warranty. The Samsung 840 Evo models come with 3 year warranty and Samsung 850 pro model comes with 10 year warranty.
For ease of mind and if you don't have the budget problem, Samsung 850 Pro would be my suggestion.
Hope this information helps.