The 850 Pro is a better drive....but I will say this - Comparisons of "real" performance - your system boots in 4.8 seconds with the 840 EVO....with the 850 Pro it will boot in 4.2 seconds....Photoshop loads in 3.5 seconds with the EVO and 3.1 seconds with the 850 Pro (these numbers are close to what I observed on my computer).
Reliability wise - if you are using the drive for a database that is constantly reading/writing, you might get your money's worth for a PC....
Most of the performance gains with a SSD are in boot times and initial program loads. Your system won't be that much faster during web surfing, utilizing programs, etc....And in most cases, it would be suggested to go with a HDD for your data - I usually purchase a 128 or 256GB SSD for most systems with a 1TB HDD.