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Samsung's made a name for itself as a manufacturer of high performance, reliable electronics, and this reputation strongly applies with its 840 and 840 Pro lines of SSDs. Although these two are Samsung's current flagship lines, the previous 830 Series generation earned numerous accolades and is still an excellent buy for users looking for speed without sacrificing stellar reliability and vice versa.
The primary difference between the 840 and the 840 Pro lines lies in the kind of NAND flash each drive uses. The regular 840 Series uses TLC NAND, which means each cell of the SSD holds three bits, as opposed to the two bits held by MLC NAND cells. So the storage density of the regular 840 Series is greater than that of the 840 Pro Series, allowing Samsung to sell it to you at a lower cost. The downside is that the 840 Series is slower and in theory has fewer write cycles (effectively lifespan) than the 840 Pro Series - but the latter concern is largely irrelevant for normal use.
A quick summary
830 Series: MLC NAND (2 bits per cell, faster, more write cycles)
840 Series: TLC NAND (3 bits per cell, cheaper, but slower and fewer write cycles)
840 Pro Series: MLC NAND (2 bits per cell, faster, more write cycles)
So, which drive should YOU get?
Honestly, if you're like most users, it won't matter. You won't reach the write cycle limits on any of these drives over the course of several years of frequent usage. You won't see an enormous difference in bootup or program load times. You'll get bragging rights if you get the 840 Pro Series for speed benchmarks, but we wouldn't advise you to buy just based on benchmarks. (Well, okay, it IS pretty cool to be able to write at well over 400MB/sec :)
That said, if you see a new Samsung 830 Series SSD at a bargain price, jump on it. It's more than fast enough for most needs and has a fantastic record where reliability is concerned. As the 840 and 840 Pro's track records become more established, chances are they'll show a similar trend, but where your data's concerned, there's no reason not to go with a proven option that often costs significantly less than the lesser known option.