The 850 EVO is using new technology where the storage is stacked 32 layers deep. End result of this is extremely fast memory access, and dramatic drops in power usage by the memory. It also uses a larger cell space which is better for an SSD's longevity.
The 840 EVO uses older single layer chips, and a smaller cell size. Smaller cell sizes in SSD storage results in shorter lifespans than larger sized cells.
The 850 is a bit faster, but is limited by the speed of the SATA III ports.
Installing Windows on an SSD provides for a much faster booting process than a hard driver can. I know on Windows 8.1, I can boot up to the login prompt in about 7 seconds.