Pretty much what was said above. Though the theoretical/synthetic frame push of dual 460's would be greater, the stability of a single more low profile card like the 750 Ti would be a dream by comparison. Again, as noted, you'd need much more wattage with at least 4x 6-pin connectors.
Also scaling was even worse then than it is now, so the benefits are few. Micro stutter will definitely be present, so your minimum frame rate will be pretty low at times. You'll definitely run hotter, and the potential for a failed card is much greater with your already used and the most likely used 460 you'll acquire.
In many cases the 750 Ti barely beats out a 260 or 275. Where the newer architecture and low power consumption shine is in stability and...