Your call. You can save $260 by going with the R9 290 but it is a 15-25% slower card (not 10-20 as joeboo pointed out).
If you want to build a $1500 rig then the 780 Ti is the better choice.
If you want to build a $1250 rig then the R9 290 is the better choice.
Overall you'd be spending about 17% more on the whole rig for a 15-25% gain in FPS. Keep in mind that gain is going from 85 FPS to 105 FPS in BF4, 134 FPS to 161 FPS in Tomb Raider, 72 FPS to 97 FPS in Hitman, and 75 FPS to 90 FPS in Metro Last Light. If you don't think you'll care about that difference and want $260 in your pocket for other things then the R9 290 is a better card. But as far as I'm concerned this is a you get what you pay for thing. Those higher FPS as new games come out will go from being those 85 vs. 105 FPS numbers above to 32 vs. 40 FPS which is much easier to tell the difference of.
And with that $300 he saves now, that will buy a card that he'll need in 2 years regardless of if he goes 290 or 780ti, that is better than either of those cards. I think that's the main point in not spending $700 on a video card now. That card won't last 4-5 years to make that price worthwhile. It's going to be just as outdaded and slow in 2 to 2.5 years as the 290 and either will need to be replaced. Spending double on the video card now isn't going to make it last twice as long.
Lets look at a theoretical 4 year period (probably the average life of a gaming PC before people start to contemplate a complete rebuild, although that is lengthening, making incremental video card upgrades even more likely)
780Ti vs 290:
Year 1 - both are great, and playing games on 1080p with ultra settings at well over 60FPS
Year 2 - This is where the 780Ti probably has the advantage, the 290 may need to start to have some settings turned down to high, rather than ultra all the way like the 780Ti
Year 3 - The 780Ti is starting to fade, it has to be turned down below ultra. Alternately, you can ditch your 290 at this point and use the $300 you saved on a totally new card that is 2 generations newer and smokes both the 780Ti and the 290
Year 4 - 780Ti is useless unless you like playing games on low settings, while the new $300-$350 card is great for a solid 2+ years.
So when comparing the same amount of money overall, ~$700 or so, the 290 + new card in 2 years scenario is probably a tie in year 1, loses in year 2, but wins in years 3 and 4. Personally, I'd chose the option that is best(or tied for it) in 3 of the 4 years, than just 1 of the 4 years.
Of course, if budget and money is absolutely no matter, then throw all that out and just throw crazy money at your gaming PC and buy a new $700-$1000 video card every 18 months to stay on the bleeding edge. I just don't see any point in the video card costing more than the rest of your build combined, that's just dumb and not the best use of money when actual real-life budgets are in play.
