i'd personally rather have two cards. that way if one breaks you're not completely screwed, not to mention they'll last longer because of not being as loaded
But why would a card break if you have adequate cooling. They are designed to downclock or even shut down when too warm.
If you are going to buy something expensive, know your stuff and treat it right.
Treating a card right doesn't always ensure it will last, like my previous cards in this system proved.
I had two old 9800GT's and they were both more than adequately cooled, my system temperature never peaks over 30c even with my new cards but one of the 9800GT's wore out after just after a year and started failing with artifacts and BSOD's
I personally agree with Waffle in the fact that it's always better to have two slightly less powerful cards, SLI has worked fine for me on any game I throw at it with great scaling, it all depends on your motherboard and full setup. Plus, a GTX 780TI starts at £549 which will still require me to do a complete system rebuild with a new CPU and Motherboard, most likely an Intel if I planned to SLI a GTX 780TI.
A GTX 770 is a pretty high-spec card and will do me fine when in SLI.