Six one way, half a dozen the other... I'd say buy the Corsair. That way, you end up with a PSU built by SeaSonic, a five year warranty, and 2A more on the 12V rail.
I'd say buy the cheaper one, they're pretty much the same quality standards as the guy above said. And incase you're wondering they output nearly the same current, 48A on the 12V rail for the M12 and 50A on the 12V rail for 620HX. I'd go for Seasonic 'cause I think their sticker looks way better than the Corsair one.
If I remember correctly the M-12 series has two fans (120mm + 60mm ??) while the HX620 only has one 120mm fan.
I would go with the HX620 since I like my PCs to be as quiet as possible. Since the Corsair is made by Seasonic, you'll be getting an excellent quality PSU.
Just a bit off topic but it's great to see a manufacturers support guru posting on a hardware site other than their own. It makes me feel all warm inside knowing I bought a Corsair PSU and their staff are dedicated enough to venture outside their normal domain.