Both are crap choices, Titan is a waste of cash because its a 780 with 'Titan' written on it, and while Crossfire is an awful lot better than it used to be I certainly wouldn't use it seeing as the Crossfire patches don't work outside DX11 and on a single screen. Better to buy a 780, or even better, wait about half an hour and see what AMD is announcing in their live stream, it will more likely be faster than the 780 for less cash.
AMD crossfire support is not good u will have a lot of lag. There is a GTX780 also which performs close to titan and cost $350 less. AMD is going to release their 9000 series so the price of 780 will come down and u may get 780 sli in ur budget.