I'm building a new PC and I am stuck between two SeaSonic power supplies; the G-Series 550W or the M12II 620W.
Between the two, the G series design is a lot more recent.
I would personally go with the higher wattage for future upgrades.
The G series has a higher efficiency rating (80+ Gold vs. 80+ Bronze) but either way, you're going to get a high quality PSU.
When it comes to a PSU i say go for broke the better PSU you buy now the less a future build will run you. High quality Seasonic for instance could last for years and with a high enough wattage it will keep up.