Your whole pc runs on less than 300w. When it's referred to gpu needs, the pc is taken into consideration. Most pc's run max around 200w ish, the only component that draws a major amount of power is the gpu. So when, for instance,its referring to a gtx1080 needing a minimum of a 550w psu it's really meaning that it's 200w for the pc, and an additional 350w for the gpu. Reality is that the gpu will in no way pull that 350w draw, probably only half that and most times the pc will not pull the full 200w, closer to half that as well, so that 550w psu is only seeing @250-300w. This puts it at running at a little over 50% capacity which is the perfect area for the highest efficiency as well as being in the best area for cooling and output. Running a psu at maximum for any length of time runs the risk of overheating, for some under quality psus it means risk of overloading and there's no room for errors.
The 750ti can run on a 300w psu, but it's preferred for a 400-450w, simply so everything has enough power, and if some part suddenly demands more there's extra to spare if needed.
Go with which ever is the better psu. Quality counts. And bigger doesn't necessarily mean better.