Neither, but depends on where you live. You should be looking at the 290 non X or GTX 780 vanilla if you want the most bang for the buck around 500$. If you're in Canada or the US the best value is actually the vanilla GTX 780 (non TI) - if you get a custom model they will generally easily overclock to 780ti [stock reference] levels of performance. Cards such as the 780 classified, lightning, DC II, and so on. All of these cards overclock well and most 780s can get 15-20% more performance by doing this. You also get Nvidia's software features which I really like - I find NV's software and features to be significantly better than AMD's - NV's software has secured my fanboydom after having used both sides for several years. But this is merely my opinion, you'll have to research features and software for both brands on your own. Grain of salt applies, my opinion only, etc.
The 290 of course also performs very well - the 290 non X would be the one to get. Outside of the US/Canada it is actually priced fairly reasonably. Keep in mind if you go the 290 route you'll want an aftermarket variety unless your case really sucks - the reference design leaves a lot to be desired IMO. But the aftermarket varieties fix this and perform great to boot.
Simply put, neither the TI or the 290X are worth the premiums over the 780 and 290, respectively. Either card should be fine in terms of performance and what you want to do. Then again, you didn't tell us what type of games you play, your resolution, and more details on your setup either. Maybe that could change things. And of course if you just don't give a crap about price then by all means go for a 780ti or 290X. You could be paying more for diminishing returns with those cards, however - while the 780 and aftermarket 290 give more bang for the buck at the 500$ level.