Honestly, you will probably not notice much a real world difference. Once my computer gets past the BIOS screen, it boots Windows in less than 5 seconds. I use the 850 Pro and have built machines using the 840 EVO. You will see a difference in synthetic benchmarks but honestly you will not see that much of a difference in real world applications to justify the price increase to the 850 Pro if you can get the 840 Evo on a killer sale.
Also, what size did you get?