the plain 840 is much faster in random reads (where it counts) and only a little faster in random writes. I'd hit the 840 as long as you have Intel or AMD chipset sata3 ports (not asmedia, marvell, jmicron, gigabyte...). If you only have sata2 or those others it wont matter so get whichever is cheaper.
Samsung it is then! Just ordered one. Thanks for your help!