1) 840 Pro has higher benchmarks - NO BIGGY as in real life You will not see a difference in day-2-day performance.
2) 840 EVO uses 19nm TLC, 840 Pro use MLC and has a higher write cycles rating - No a bigg issue as under normal usage the your still looking at 10+ years of use.
3) Power consumption very close for EVO and Pro ( about the best for SSDs)
... Power consumption really only an issue for laptops.
4) Pro = 6 gigs more space - again depending on space required probably not a biggy.
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To say that I really like the EVO is an understatement. If Samsung can keep quantities of the 840 EVO flowing, and keep prices at or below its MSRP, it'll be a real winner and probably my pick for best mainstream SSD.
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Ref:
http://www.anandtech.com/show/7173/samsung-ssd-840-evo-review-120gb-250gb-500gb-750gb-1tb-models-tested
Page 1 has a good discussion of TLC NAND
Use the pull down to look at benchmarks and power consumption.
Added: based on comments that popped up while writing this.
Based on Price the Pro is the clear winner.
More on Benchmarks: I have Crucial M4, Samsung 830s and Samsung 840 Pro. Using AS SSD as benchmark, the 840 Pro should (REPEAT should) be about 60 % faster - YEA Right There is VERY little difference in real life, hard to tell which system has which of the 3 SSDs installed.
M4 and 830's Overall score approx mid 700s, 840 Pro scores a wooping 1100. BUT I love my 840 Pro.