What price are you getting the X-750 for..? On newegg, the Corsair HX 850 is available at a price slightly lower compared to the price of the Seasonic X-750.. If you wish to stick with a 750 watter then getting the Corsair HX 750 or the XFX 750W Black Edition are extremely high quality units..
To answer the OP's question, as others have said the Seasonic is in a league above the Corsair (which is a good PSU itself). If there's anything else out there as good, it was probably built by Seasonic using the same platform.