Depends.... 750Ti's are good at the moment but there is not much to mine with them so if the few profitable coins die they will be useless. Much more stuff to mine with 280x's and if you get them used they can be a good deal. I have a lot of 280x's and some 750Ti's and I can tell you that 750Ti's dont have much of an advantage in powersavings actually. If you know what you are doing you can get your 280x's running at 0.756V and they will be on par with the 750Ti  on energy costs still pumping out more Khashes.

If you are just starting it's probably not the best time to enter anyway...