This choice should be obvious. More obvious if you don’t own either. Manx Loaghtan is discontinued and you can only get it used unless you get extremely lucky. I am not 100% but I assume Catalinbread hasn’t made these pedals same time. Manx Loaghtan is like Big Muff with two band EQ. Giygas isn’t exactly that but you can get quite close to Manx Loaghtan. But only quite close.
Putting them side by side with high sustain/fuzz Manx Loaghtan sounds better to me. Not much but it does. Could be because I have had it much longer than Giygas. I can get quite close to that with Giygas. Probably if not comparing side by side I would be happy with what I get from Giygas. Giygas doesn’t sound as sweet and smooth. I found sound I like from Manx Loaghtan and tried to get same sound from Giygas only getting close but not quite there.
You can probably see the problem. I tried to get same sound from new pedal old pedal was really good at. When I changed the setting and tried to find sound I liked from Giygas and tried to get same from Manx Loaghtan Giygas started to feel superior. Which it is. It is more versatile. Manx Loaghtan is really good at one thing. Sweet and smooth high gain sound. Giygas is better at almost everything else thanks to blend knob. I will probably put Giygas on pedalboard. I can get more good sounds from it. I can always put Manx Loaghtan before pedalboard if I need that sweet sweet fuzz sound and I can’t get it from Giygas.
Manx Loaghtan’s knobs have more usable range. With Giygas I have EQ always almost the same position. Loud, Blend and Fuzz knobs knobs move more often. I put two videos showing pedals. Giygas video shows the versatility of the pedal. Pedals might not sound so similar in the videos but I tend to get quite similar sounds from different kinds of pedals. That is why I will have only one of these on pedalboard. Giygas video doesn’t try to make it sound like Manx Loaghtan. You can almost get there if you put them through same effects.