As long as we are talking about the 840 and not the 840 pro, the 830 is generally regarded as more reliable than the 840. And I don't feel that the 840 pro price leap is justified.
The 840 uses cheaper tlc nand whereas the 830 uses better quality mlc nand. I'd go 830 over 840, but the price difference over here is like $10 so it doesn't make that much difference - unsure on the price difference in your part of the world.
In regards to fitting your laptop I am under the assumption that any ssd will fit but don't quote me on that, I'm sure someone will be able to clarify.