560ti will be a bit faster but are you willing to pay 100 bucks more for that. On the other hand buying a 560ti now and maybe put another one in over a year or so gives you def. more powerrrrrrrrrrrrrrrrr.
For the moment, you should probably get another gts 450 to save money and make sure your PSU can support it with another 12v rail and at least around 600 watts. If your PSU isn't good enough, go for the 560 ti.