I have both lenses and would take the Samyang under most circumstances for what I do ( landscape photography).
I bought the FA 20mm years ago, and enjoyed it on APS-C until I bought the Sigma 18-35 and the FA 20 was largely relegated to storage - the Sigma was sharper and more convenient for covering multiple equivalent focal lengths.
When I upgraded to the K-1 I later bought the Samyang 20mm to fill the gap between my Sammy 14mm ( already owned from when I had APS-C ) and the Sammy 24mm ( early lens purchase with K-1).
Shortly after the purchase I did do a quick comparison between the FA 20 and the Samyang ( standard brick-wall test
), and the FA 20 was less sharp on the edges - not necessarily soft, just not as good.
The Samyang is a good lens, but I still find myself having somewhat mixed feelings about it. It's not quite as sharp as the Samyang 24mm stopped down ( at least my copy ), but it's certainly no slouch. The corners are a bit softer, and sometimes when I look at photos ( translation: pixel peep
) I wish it was sharper, and other times I'm thrilled at how sharp it can be ( the problem is probably me rather than the lens! ). I haven't used the D-FA 15-30, but from what I've read about it I wouldn't expect the Samyang to produce anything that is necessarily sharper. The opportunity to use screw-on filters is fantastic, though; but be warned - stacking filters will give some very pronounced vignetting. CA's can be quite strong and will be a bother in higher contrast situations. I can't give much information regarding distortion, as I don't shoot architecture very often and most things in nature ( except perhaps horizons ) aren't usually straight. However, I recall the FA 20 having very pronounced "mustache" distortion that will be quite noticeable in photos with flat horizons, while I haven't noticed anything obvious with the Samyang.
One thing I have noticed that is worth mentioning is a rather strange colour-shift that occurs with both my Samyang 14mm & 20mm lenses. Near the edge of the photo ( and especially the corners ) I will tend to see a green-yellow tint. It's bizarre, and only seems to affect these lenses out of my kit. I expect that it's because they're ultra-wide?? Not entirely sure, but I would be interested to know whether this shows up on other UWA Samyang lenses or other ultra-wides in general on the K-1 ( maybe it's just my copy?! ). It's a manageable issue that usually shows up on skies, and is usually fixed with some radial filters in Lightroom and adjustments to the White Balance of aforementioned filters.
As for wide-open performance of the Samyang, I can't actually comment with any sort of authority
. The last time I remember shooting it at f1.8 was during my initial tests for decentering. I usually shoot stopped down to f8 or f10, and I I can't remember the last time I pointed it at the night sky - I usually used longer lenses and stitch for my nightscape work. If I had the opportunity to photograph an Aurora, the Samyang would be my first choice - but if you're going to Iceland in summer I guess that means you'll have a midnight sun!?
Can't really comment regarding flare. Both probably have it, but I don't point it at the sun very often.
Auto-focus is +1 for the FA 20, but I don't really miss auto-focus as most of what I do is off a tripod. My Samyang 20mm also has distance markings that are actually accurate and a hard stop just a fraction beyond infinity! The 14 & 24 go to infinity and well beyond, and the distance markings on the 14mm are little more than drunk suggestions
So, the Samyang is in my field kit most of the time, and although it's far from my most used lens ( possibly not even Top 5
) I've taken some of my more favourite photos with it. Yeah, it's heavier; but the compromise for the extra IQ is worth the weight for me. The FA 20 is in storage, probably until I stop caring about pixel-peeping and look for something with a bit more character, or need something that is much lighter and smaller. Never say never
Some shots from the Samyang 20mm f1.8 and 1:1 crops to show centre,side & corner sharpness, and uncorrected CAs. Please note that the pic with the coastal flowers is focus-stacked from 8 different photos - the lens is good, but it can't work magic with the depth of field needed for that.