Comparing these gets very nitty-gritty, so first I'm going to post the sources of my information.
Seasonic G Series 550W Review:
@Jonnyguru
EVGA 550 G2 review:
@Jonnyguru and
@Tomshardware
Now, the 550 G2 does do better in two areas, very slightly. That would be ripple and noise suppression, and voltage regulation. The Seasonic comes close, but does not quite match in this case. There is, however, one big downfall of the 550 G2. When there is an AC loss, the voltage outputs drop. Once the 12V rail drops to 11.4V, which is the minimum per the ATX specification, it is supposed to cut off the PWR_OK signal to the motherboard, and about 1ms later shut off. The problem with the 550 G2 is that it waits until the 12V voltage is all the way down to 10.8V before dropping this signal, which is bad and can cause stressful effects on other hardware. How do I know this? It's on the Superflower Leadex Gold 550W review
@Tomshardware - the unit the 550 G2 is based off. With that, the 550 G2 does not even meet the ATX specification for hold-up time (time from AC power loss to PSU shut down), which is 17ms. It gets 12ms, that is with the voltages far out of spec, so this is really poor performance in this aspect.