Hello,
The Samsung 830 is great. Well tested now. Last firmware update was last January so it's stable.
The 840 looks promising, but because it has no track record yet, is more expensive, and has an avg read rate of 540 vs the 830's 520 MB/s, I'd go with the 256GB Samsung 830.
Remember it come in three varieties, full kit for desktop upgrade with bracket etc, bare drive, and the laptop upgrade kit.