The G550 is a way better unit (tight voltage regulation and ripple suppression). Plus, it's 80+ Gold and a newer unit so it's bound to be more expensive than the M12II. But, if you don't really mind those things (but you should), you can't go wrong with the M12II or the XFX one.
What matter is the quality of the psu and its electrical performance, of course, if you find a cheap quality psu with plus gold certificate, it's* a chance not to be missed, the Seasonic G550 is at a good price and it's cheaper than the m12ii 620.
Pcpartpicker tells me it's around 80-90$ : http://pcpartpicker.com/part/seasonic-power-supply-ssr5...
I always thought the efficiency was great in terms of the decrease in heat generated. The higher the efficiency the less heat generated inside the case (and less heat that surrounds the GPU, which is already hot). Sounds good to me.
If the op wants a modular psu(the plus gold certification comes as an extra), the seasonic G550 is a good choice, if he doesn't, the xfx 550 core edition is cheaper, with still good efficiency levels.
Also, it might be useful to know what kind of system it will be powering, also if you plan to add a SLI/Crossfire of x or y card.