The CX550M has about 70W more on the +12V rail. Seasonic has only 480W. Corsair one would allow more upgradability, a beefier GPU, for example. Then again, Seasonic uses better internal parts, such as all Japanese capacitors. To be honest, can't go wrong with either of them, so I can just say - pick whichever is cheaper from the two. Another plus for the Seasonic unit is that it is fully modular, which means good news for cable management. M12II's are about 8 years old now, think they were released in 2010 or 09. Grey label CXM is a newer PSU. Both come with 5 year warranty.
Oh, and I checked both PSU's, the Seasonic variant is 50 cents cheaper (the 620W variant, not 520) than the CX550W. About $60 on US Amazon.