Many people ask when they upgrade to an SSD if they should get a Samsung 840 or the 840 Pro and want to know the difference. I usually say that unless they are in a production/enterprise environment where they will be doing excessive writes to it daily, then they don't need the Pro version.
Samsung Electronics Samsung 840 Series Solid State Drive (SSD) 500 GB SATAIII 2.5-Inch MZ-7TD500BW
The NAND on the non-Pro is made from TLC while the Pro version uses the more robust and durable MLC NAND. The MLC allows the Pro to have a better write speed and stands up to more writes than the non-Pro before degradation begins to occur.
If all you're looking for is a faster boot time and faster application load times, then the non-Pro is all you need. The read speeds between them is negligible and would only be noticed in benchmark tests.
Are you worried about the non-Pro wearing out too soon? The non-Pro is designed to last at least eleven years while writing 10GiB per day with a write amplification of 3x. After eleven years, it'll likely be obsolete along with the computer it's in before the NAND starts to degrade.
As you can see, from the testing done at Anandtech, the read speeds are very similar even with the 830 included. The write speeds are drastically different, but as I mentioned before, unless you do a lot of heavy writing each day, one doesn't need the Pro version.
I'm all for top-level performance, but when the prices of SSD's are still very high compared to their much slower counterpart, the hard drive, you're going to pay an even higher premium for the Pro version. For the average user, the Pro version is just a waste of money when they won't be exercising its full capabilities for cost.
Here is a speed test of the Samsung 830 in my MacBook Pro, which I used to write this article. Even as a non-Pro model, it screams speed and even boots to a useable desktop in 15 seconds.
The bottom line is, if you're on a budget and don't intend on heavily writing to your SSD daily, then go for the non-Pro 840. However, if money is no object, then get the 840 Pro and enjoy the faster write speed.
Download the pdf version with pictures here.