like a lot of PC hardware the name on the box is just a brand and not the company that actually made the product, better know as the original equipment manufacturer or OEM . Both of those PSU are made by a company called Channel Well Technology (CWT). While both PSU are labeled as 600w, the GS600 can deliver 48a (higher the better) on its 12v rail (powers the CPU, GPU, HDD, etc.,) compared to the CX600 which only offers 40a. The GS600 also uses a few better capacitors then the CX600 for a slightly better internal build.
corsair plans to introduce a new version of the GS series called the V2 but I'm not sure when they hit the market. A few improvements are made to the previous platform for better efficiency.
http://www.corsair.com/blog/corsair-gs-series-psu-makeover/
review of the new GS800 v2
http://www.techpowerup.com/reviews/Corsair/GS800_V2/
either PSU will do the job as you will be around 300w while gaming, of the two I would go with the GS600 if I could get a good price on it. otherwise, the CX600 is a fine budget PSU.