SLC memory is usually more expensive, but it would allow a small 8-16GB disk to be a viable option for a desktop system drive. Its prefered over the 'bad' cheap MLC SSDs without a good controller. So:
Worst option: cheap MLC SSD without an advanced controller
Good option: cheap SLC SSD no larger than 16GB without an advanced controller
Best option: MLC SSD with good controller (Intel, Indilinx)
The performance of an SSD relies heavily on the controller used; without a good controller, MLC SSD performs very bad. With SLC memory you can get away with a bad or simple controller.