Not sure if a 2x 500GB setup would be better/cheaper than a single 1TB SSD, but the 840 EVO 1TB seems pretty compelling, cheaper than two 840EVO 500GB, not sure if theres a compromise somewhere though. I want to replace HDD storage outright, I don't store anything on local storage I could afford to lose, pretty much 100% for games and programs.
Using Win 7 and an Asrock Extreme 4 Z77 if that matters.
What are the possible cons that you are looking for, given that you say the price for the single larger drive is cheaper than the two smaller ones - and that you'd do amazingly well to saturate a SATA3 port?
The downside of what a similar setup on traditional platter disks is that putting your OS and Data all on one drive means that the same read heads are doing more work than they would just running the OS and programs. SSD's are astoundingly faster at read/write/seek operations, so it seems unlikely that you would notice any real world difference by loading up your OS drive with data. Games and many programs rely heavily on RAM to function, which will means that your active "trouble" spots will only be loading games/menus ect. I have no data to support my guess, but the performance of the SSD you mentioned is excellent when compared to most (might I suggest ALL) modern SSD's. When compared to a traditional HDD, the comparison almost becomes unfair.
The only possible downside that I can think of, is that you say that you cannot afford to lose any data. If your 1TB SSD fails for some reason, you lose 1TB. If one of your two 500GB SSDs fail, you lose 500GB.