I have been doing a lot of research on this recently, and though I am no expert, I believe i can provide something of a decent explanation on this one. I welcome any gentlemanly retorts:
The main difference, aside from any easily referenced read/write specs, is that the 830 series is MLC and the 840 (non-pro) is TLC.
TLC basically sacrifices (theoretically) some measure of longevity (relative to MLC and SLC), in order to provide a lower cost/GB level.
The easiest way to explain that (if you don't already know the difference) is TLC = X*3 , and MLC = X*2 , where X equals the initial capacity of the physical cells (which is what SLC is, it uses no multplier, and is therefore typically the most expensive of the 3 types). X costs what X costs to the manufacturer, and from there it is just what algorithm you want to use to create the effective capacity of the drive. The sacrifice is the more levels you add, the more wear it puts on the cells.
However, how consequential this is depends on your use. I have had much trouble determining what the average "enthusiast" builder (basically gamer, web surfer, other general things, but not a server administrator etc....) uses per day, but the tentative consensus is that it basically shouldn't concern you. The 840 comes with 3 years warranty, and even based on the equations people have come up with, it should last longer anyway.
My opinion, which of course will be tested over the coming years, was that the discounted price bracket that the TLC tech provides is worth it, so with my new build I just assembled last week, i ended up going with the SAMSUNG 840 120GB, after what i consider to be prudent research into the matter.
In conclusion, IMHO, it is a fine drive, and samsung is a good company, so take the deal, unless you are making 1TB of writes per day or something.