I might go with the Seasonic because of the assumed better component quality. I assume that a higher rated PSU (all other things being equal) will be quieter. What I would be sacrificing is low-power efficiency, right?
All those assumptions are, unfortunately, wrong. IMO, of course.
Seasonic made up its name in the last decade, so that nowadays, even when it isn't up to the past glory days, they can bank on that name to stand out over the competition: just like IBM in the seventies, no one has never been fired to have bought an IBM product, even when IBM wasn't the best candidate.
As ODM Delta and Flextronics are better, as well probably Etasis and Zippy (not used in consumer markets).
As OEMs better than any current Seasonic are again Delta (Antec) and Flextronics (Corsair).
At the same level of Seasonic there's the chinese "nouvelle vague", like Super Flower, Great Wall and Chicony. Impervio-Enermax at his best is at least on par with Seasonic, but now their best unit are outsourced mostly to CWT.
Almost always the top end units made by ATNG (Fractal), CWT (Corsair, Enermax), Enhance (Corsair, Cooler Master), Sirfa (Thermaltake DPS) are as good as the Seasonics, and when an ODM is willing to pay the requested high prices for their high standard QC processes, even AcBel and Huntkey may deliver better products than current Seasonics, build-wise.
Summarizing, buying a Seasonic today is just a shortcut to avoid verifying actual figures, sort of a "common wisdom" to not taking care to check whether the promises are fulfilled. Don't get me wrong: Seasonic are very solid units but today, alyhough they're among my favourites, they're not necessarily the best ones, period.
With reference to your requirements, at your expected power draw (I mean now, without any just foreseable discrete graphics) you may think that a very cheap Corsair CX-430 is somehow better than a Seasonic Platinum 660 in "normal" mode: up to 80-100W the 12V regulation is better than 0.5% for the Corsair vs the 0.6% of the Seasonic, the 12V ripple is around 10mV for the Corsair vs around 20mV for the Seasonic, and the Corsair would run a tad quieter. The cheap parts inside the CX won't be stressed by that low power level, so that you will likely retire it from service before it fail. And here in Italy for the same money of a P660 (128 euros) you would buy almost four CX430 (36 euros).
Set aside this sort of joke, you wrote: "First on my list is to find a PSU of high quality (wrt. efficiency, durability and performance). I'd also like the PSU to be quiet, that's one of the reasons I'm here
" and I hold on my previous recommedations.
Quality-wise and reliability-wise Platimax, Dark Power, Golden King and derivatives are at least as good as the Seasonics. The Straight Power is not, it sports a less advanced design (as the Xilenser/Dark Power) and too many second-tier parts (however it has a comfortably long warranty). It might worth to note that the latest Seasonics seem to suffer of quality control/production run issues, with a subsequent RMA ratio in excess of the one of other high quality platforms in the market.
Performance wise the Seasonics XP3/KM3 may have a slight edge on the Golden King, and a more consistent margin over Enermax and BeQuiet: IMO/IME those margins don't mean anything better for the Seasonics either in absolute terms, or even more with reference to the expected very low power draw.
Noise wise Platimax, Dark Power and Golden King derivatives are surely better than any fanned Seasonic, either in hybrid or normal mode: they will run cooler than a Seasonic in hybrid cooling, and they should last longer than a Seasonic in hybrid cooling. With the midrange Straight Power E10 your mileage may vary but I'm fairly confident that at least up to 200W DC it will be quieter than any fanned XP3 Platinum or KM3 X-series (which are the same PSU, more or less).
Money wise, noise wise and efficiency wise your best bet is IMHO the Straight Power E10 400W, and without any discrete graphics the inner quality is maybe still overabundant to achieve a long term reliability.
Second best, money wise, quality wise and noise wise is the Platimax (but as many first-time Platinum unit it's not that efficient at your very low loads, and however marginally less efficient than latest Seasonics at any power level).
And with reference to your last assumption "a higher rated PSU (all other things being equal) will be quieter
", that's quite the contrary: often that's false, and that's false particularly for latest Seasonics (with the >1kW units which are rather noisy in absolute terms, IMO).