I personally would go with the 500GB 840. My choices were initially the 840 Pro, OCZ Vector then was swayed to the M4 and now the 840.
500GB 840 at $310 is
$0.62/GB. 256GB 840 Pro is $270 at
$1.05/GB.
It has the 2nd lowest at-idle power consumption trailing the 840 Pro by a very narrow margin. This is important as your computer will be at-idle most of the time.
As you may have noticed people who owned a Crucial M4 upgraded to the 840. 512GB M4 is $385 at
$0.75/GB while the 256GB M4 is $190 @
$0.74. If the 840 is not available in your area I'd get the M4.
If you are coming from any HDD any SSD will be faster. Any SSDs will consume less power than any HDD. This was how I was swayed to think that I will not notice the performance gap between the M4 and 840 Pro. I'm not saying the 840 Pro's not a worthy buy. If you want the best and you can buy the best then the 840 Pro's the way. I initially went for that didn't I??
Take note that we are a far a ways of saturating SATA 3Gb/s (300 MB/s) & SATA 6Gb/s (600 MB/s) write and read. So even if you got the top of the line SSD today you there will always be a drive in the future that will be faster for that particular SATA.
Now for the comparisons.
Included is a Hybrid Drive for comparison.
Write at SATA 6Gb/s (600 MB/s)
066.2MB/s - Seagate Momentus XT
314.1MB/s - 500GB 840
460.0MB/s - 256GB 840 Pro
262.8MB/s - 512GB M4
242.2MB/s - 256GB M4
Read at SATA 6Gb/s (600 MB/s)
076.3MB/s - Seagate Momentus XT
506.6MB/s - 500GB 840
515.0MB/s - 256GB 840 Pro
511.1MB/s - 512GB M4
472.0MB/s - 256GB M4
Source:
https://forums.macrumors.com/posts/16503046/
https://forums.macrumors.com/attachment.php?attachmentid=383999&d=1355702348
https://forums.macrumors.com/posts/16528913/
The 840 has 3 years warranty vs the 840 Pro's 5 years. I think of it this way. When the warranty expires after 3 years time SSDs will have double the capacity for half the prevailing price. So by then I can get a 1TB SSD for $155 at a write/read that may be better than a 840 Pro.
Alternatively 3 years from today you will be buying a whole new MBPro with Retina Display. Those comes standard with SSDs.