You most certainly get to choose which tech you go with as far as SLC, MLC, TLC goes. That's complete nonsense. Your 830 uses MLC btw, so its as simple as saying I'm going to buy and 830 over an 840.
SLC, MLC, TLC refer to the single layer, multi-layer (double), and triple layer nand memory used in SSDs. SLC stores 1 bit, MLC 2 bits, TLC 3 bits. Because of the larger cells in TLC, they are prone to wearing out quicker, which will eventually be overcome by the chip manufacturing process. Hell, the current manufacturing process could be sufficient enough, but that's the big question isn't it.
If you want a better understanding of TLC and your options, Anand wrote an decent article a while back that you can read: