840 pro are better drives, this site actually prefers a 840 pro 128gb over a standard 840 at 256gb
there are faster write speeds on the 840 pro and they are supposedly more reliable and last longer.
in fact i bought one yesterday
there are methods of making your users folder and program files folder to be moved to a secondary mechanical hard drive so you wont use to much of that space.
all in all the 840 pro is more quality.