If you are generally working with something that requires writing alot with your drive then you will see a huge difference, people keep saying it only shows up in benchmarks, no.... When writing huge files and you wanna save more than half the time on writing that then the pro's 450 mb/s is much superior to the normal 840's 130 mb/s.
If were talking about games, then the 840 and pro have no difference in load times. 256 GB Samsung 840 Pro is around 20 $ more for the cost of faster writing speed and 6 GB storage and extra 2 years warranty. I would get it anytime.
But it's very few that requires the extra write speed, and the prices on the 250 gb and 256 pro is around 60-70 $ in the US. Simply not worth the extra warranty and 6 gb storage. If your like me from Denmark then the Pro is the better choice.