Samsung is better in nearly all areas. People saying the Crucial is better are mistaken.
That being said, if the M4 is a bit cheaper or the 840 is out of stock, it's still a good deal. Fast and stable.
That's a dumb statement. 840 has only been out a couple months, how the hell is it more reliable than the m4 which has been considered one of the most reliable for years. Numbers don't really means much between ssd