S&P Downgrade of Banks

Standard & Poors has downgraded many of the banks that issue Cdx3 preferred stocks. How does this downgrade affect the preferreds they issue? It seems that there was quite a sell-off, is it overdone? Do you consider the Standard & Poors ratings or just Moody's? I believe one bank, Citizen's is now below investment grade per S&P. Your thoughts would be appreciated.