The CM Silent Pro will get you the most stable power output, it's Gold rated as well so it bears 90% efficiency throughout the spectrum.
The GS600 is only bronze rated (80%) so you're looking at a shorter period of time before the output voltage of the PSU drops below that which the PC needs. (As PSUs age their output dips). From what I hear it also delivers a less stable voltage, with fluttering on the 12v.
The GX2 is a reasonable choice, plagued by the same issues of bronze rating. It's better than the GS600. I would recommend this if you didn't have a Haswell as it's probably the most rock solid build PSU for OCing, and the extra 50w of power would help, even if that only translates to an effective increase of a few watts (take away 10% of the power for the lower efficiency).
All in all, try for the Silent Pro. It will serve you well, last longer, and require a little less power for the same job saving you on your bill.