Ok, after doing some research, it seems the SeaSonic is a better power supply. I'll go with that one!
I have the Seasonic 1250w myself and I love it.
The XFX ProSeries 1250W Black Edition uses the same internal design as the Seasonic X-1250. The only difference between them, apart from appearance is that the Seasonic uses a 12cm Sanyo Denki San Ace, Ball bearing and the XFX uses a 13.5cm Protechnic Electric MGA13512YF-O25, FDB fan.
I guess that's why the XFX costs $2 more.
So does that mean the XFX is better?
No it means that XFX is a Seasonic with a XFX wrapper.