CYB is more technical than the Marin & Penmachno. It’s also about 1hr:15min away from Llandudno.
Fitness wise Penmachno Loop 1 is roughly equivalent to the Marin (calories burned). Penmachno loop 2 is quite flat and pedally. Also for some reason Penmachno seems to hold water on the track longer than the Marin – so if it’s been raining recently you’ll get a proper soaking at Penmachno whereas the Marin drains better. (But getting wet and muddy is part of biking!)
I’d go for the Marin, ok the climbs are mostly forestry roads but there are plenty of single tracks with a very good last descent to the car park.
If they have some experience have a stab at Penmachno Loop 1, probably a little bit more technical then the Marin but nonetheless a great blast.