The Corsar Builder Series CX500 is a good value-oriented PSU. The S12II-520 is a phenomenal, top-tier PSU with two more years of warranty and outstanding power...it is - hands-down - the better qualiy PSU between the two. Review from a top PSU review site: http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story&reid=185
That being said, the Corsair is less expensive and a decent quality PSU. I wold't have an ssue recommending either PSU depending on the circumstances. If it were my money and I could afford the S12II-520 then that is the route I would go. I have the modular version of that PSU and it's awesome.