If you can get a 290X for the $550 release price it's good value. If you can wait for the Asus DCII or the Gigabyte WF3, which have been previewed so should be available any time now, that's even a better choice, IMO.
Except, no, it's not. The GTX 780 when overclocked is 15-20% faster than uber mode 290X, and the 290X costs 600$ on newegg while the vanilla 780 costs 500$ with three free games. There is no situation in which the 290X is better, since the overclocked 780 is faster, quieter, and has a better software ecosystem.
The OP mentioned nothing of value, he asked a specific question about 770SLI versus 780ti. I fail to see where Radeons enter the question? But if he wants value, the GTX 780 is the far better option (100$ cheaper than 290X *and* much faster once OC'ed) by being better by every metric and significantly faster once OC'ed. If he opts for a 780ti, that is a good deal faster than the 780 still.
For instance, let's take this overclocked GTX 780:
This 780 is ~20% faster than the Titan.
The 290X in ubder mode is EVEN with Titan
Let's review. An overclocked vanilla 780 is
faster than the 290X by 20%, quieter, 100$ cheaper, and has better software. I fail to see how any reasonable person would even consider the hot and loud reference 290X which has inferior overclocking ability (although, custom PCB 290X cards *could* change this) and is more expensive.