I think the 850 EVO is a safe choice. Personally, I'm so totally p*ssed off at how Samsung handled the 840 EVO that I have been buying Crucial SSDs since then but that's mostly just being really annoyed at Samsung.
Just google "840 evo firmware issue" and you'll see that Samsung totally botched the 840 EVO, they have issued two firmware updates when what Samsung truly should have done was recall the 840 EVO and issue every owner with a replacement 850 EVO.
Whatever you do, do NOT touch the 840 EVO with a barge pole....