I did both last week, different days. Penmachno was pretty dry and very quite, didn’t see a solo all afternoon. The Marin has som diversions at the moment and you miss the top of the first climb and some nice down and also some of the last climb and one of the last 3 downs, which is a shame.
I like both, Marin does have some downhill fire road, which annoy me, but the final downs more than make up for it. I did both loops of Penmachno last week, but tend to miss the new one off normally as its a bit more XC (IMO).
At the moment I’d do Pen with out the new loop and the Marin. Weather is a key consideration, if it’s pissing down or windy I’d avoid Penmachno, as it pretty exposed in places, the Marin handles the bad weather much better. Enjoy.