The 840 pro is old and discontinued. The 850 pro replaced the 840 pro.
I can tell you from experience that the 840 pro runs really hot and drains the battery pretty hard. At least it did on my other mac. I had to swap it out to an intel 520 SSD and put the samsung 840 pro one in my PC.
I'd recommend the 840-evo - it's cheaper, still pretty fast, and you'll get a long run out of it. The 850 pro I believe runs cooler. You might want to do some reseach as i'm not positive.
840-evo vs 850 pro is what you're looking to research. To be honest Intel has a pretty solid SSD too so don't shy away from them.
Samsung or Intel - both make solid SSD's. I don't trust anyone else. Well maybe seagate - I'd have to read reviews tho.
Also make sure you download trim enabler at
http://www.cindori.org/software/trimenabler/
Trim is super important in SSD's.
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Amazon has the 840-evo for 88 bucks
http://www.amazon.com/Samsung-Elect...F8&qid=1408592538&sr=1-1&keywords=samsung+evo