sangram said:One: Seasonic (Corsair) vs. Topower. Search for both. Former is OEM for Corsair HX series, latter for Tagan BZ series. Seasonic has a much more solid reputation. Topower is second rung. Not bad, but not the best.
Two: Corsair units generally have far less ripple on their rails at full load than the Tagans.
Three: Line and load regulation of Corsair is superior to Tagan units.
Four: Component layout and selection in the Corsair is well above the Tagans.
Source for 2, 3 and 4 - start reading some decent hardware sites. [H] has a decent test methodology and so does PCper, both measure PSU performance at various load and line conditions, as well as check for noise and PCper even does crossloading tests.
The HX620 punches well above its range - PCper has tested the VX and the TX range, and the HX is supposedly superior to both of them. The BZ is probably capable of reaching its load specification, but we do not know how stressed the unit will be. If the various reviews on the web are to be believed, the ABS/Tagan units push things to their limit to reach operating spec, and sometimes go out of spec when pushed hard.
BTW most high power PS today are single rail, even though the spec may state differently. The Tagan units are for sure single rail, but claim to be multiple rail. I'm not sure of units that hide their true specifications from the consumer, as a simple opening of the unit confirms that the BZ800 and 1300 are both single rail. Not cared to find any reviews of the 700, but you may try a search.
Is the Tagan worth the extra money? Yes, if you are sure your load will be about 50 watts more than the Corsair can handle. You haven't even specified your graphic card, and there isn't one on the planet the 620 can't handle. If you have two, then even the BZ700 is not enough, you need 800 watts, more if you want room for overclocking.
And I don't agree with the statement that 700 watts is better than 600. See my main rig spec, it's a lot more power hungry than yours (older hardware + 8x80mm fans) at least at stock everything - and it all runs off a True Power 480 from three years ago. Rails are rock solid and quiet, though the supply gets a little warm at times. Quality trumps quantity every time, and don't let anyone tell you any different.
But you'll never take a good decision with your head. If you got your heart set on the Tagan, go for it, at least you got what you wanted. And I for one am not saying it's a bad supply.
I think both are overkill anyhow for your rig, a single card setup with a couple of drives and a modern CPU at stock will not draw more than 300 watts from the supply, so a good 400 watter should be able to cope with 90% of the systems out there. Once you start overclocking and adding more cards, SLI/CF or other goodies, is when supplies start straining.
Have fun whichever one you choose.