Normally I wouldn't be so picky about selecting a PSU (I make quite a lot of systems for others), but because this is my first PSU-upgrade in years, I want to be as sure as possible about my decision.
I understand your feelings but I think you should write down something about the expectations to meet
You had a 430W moderately efficient M12-II and you're quite satisfied of it: apparently you don't need any more power (so any 400W should be plentiful), and you like the noise-signature of your M12, which is quiet but it isn't nor a particularly silent unit, neither a spectacular performing one.
Which thing do you want to ameliorate? The Seasonic X-650 KM3 is 50% more powerful (but you don't need it), it's far more stable, it's silent, much more quiet than the M12 you're used to, so it comes at a cost. The Seasonic 460FL2 F3 (or the slightly less powerful 400FL2 F3) is actually a 80plus Platinum unit, and it's even better for silence and stability: with no power gain (or a slight loss) over your old PSU, you trade in a bit more efficiency for this fact, and about no discount over the X-650.
The G-550 is about as stable and efficient as the X-650, it gives you 120W more than your M12 (and, again, you don't need it), while it's not silent: noise-wise it should be comparable with your M12, but with a notable edge margin under the 150W mark; for the added noise over the X-series, you got a sensible discount from Seasonic.
I just point you out that you may look around for the Cooler Master Vanguard-series: the V700 is a clone of the X-650 (they shares the very same KM3 platform), and here in Italy it costs about 20% less than the original Seasonic unit, but actually clearing the 80plus Platinum requirements (and apparently ready for the upcoming 80plus Titanium specs).
The greater efficiency pays for itself, but do you need to spend more for the more stability? And do you need the more quietness if you're satisfied of the M12 behaviour? You know the answers, they depend from your perspective.
I will add just my personal experience with the X-series: I swapped an M12D with it, and I was pleasantly surprised by the slight change with reference to quietness; but when I compared it with a (relatively) cheap Enermax Eco80 350W, even if this latter is quite an inferior PSU, I have had to admit that the noise change was probably not economically justifiable (as the Seasonic X was three times more expensive). Please take note that the Enermax is anything but underpowered, for several months it played my folding rig, a Phenom II X3 740 Black Edition with a pair of Gigabyte GTX 460 OC 1GB.