MLC stands for Multi-Level Cell and SLC obviously Single Level Cell, but what exactly are these Single-Level Cells store one bit per cell and Multi-Level store typically three or four bits. The problem with this is that if a bit fails, it takes down all three bits in the cell. MLCs are slower in read and write operations and use more power. The advantages, though, are that MLCs offer higher capacities for the same price, the trade-off being their higher wear rate when compared with SLC technology.
All flash memory suffers from ‘wear’ which occurs because of the voltage applied to each cell. Charge causes a shift in that cells characteristics. Eventually this is manifested as a failed cell. Typically a Single-Level Cell will last ten times that of a Multi-Level Cell.
Wear levelling techniques ensure that cells wear evenly and redundancy reserves a portion of the device’s capacity to replace failed cells. MLC has become much more developed in these techniques, to offset their failings. eMLC or Enterprise Multi Level Cell is optimised for the enterprise sector and has better performance and reliability.
Now you know the difference between different types of solid state storage, it is worth considering which best fits your needs. Clearly from what we've discusssed so far, SLC is much more robust and reliable. You are less likely to experience data recovery issues - the downside is the price tag. MLC is not as reliable but wear levelling techniques are compensating for this. So we would advise that unless your hard drive is getting heavy usage, opt for MLC, as the reliability of an SLC would probably be wasted on you.