Excluding the cost, I would choose Galápagos, which is more bio-diverse. You won’t see penguins, marine iguanas, mola-mola, Sealions in Cocos.
During this pandemic situation, Cocos gets my vote due to short (< 4hrs) nonstop flight and being more sharky. I tried to get to Galápagos right after Christmas with the 50% off offers by some liveaboards and had to give it up trying with long layover international flights in Panama & Guayaquil with total travel time of > 11 hours and no available domestic flight to GPS.
34 hour sailing to Cocos in December 2020 with Okeanos Aggressor 1 wasn’t as pleasant as with Argo in September 2018. May be the weather change (from raining to dry season) in December is rougher than in September (raining season). Okeanos Aggressor 1 may also be not as stable as Argo. The constant side-to-side swing during the crossing from Puntarenas to Cocos on Okeanos Aggressor 1 made me sea sick. The sea sickness medicine did not help. It was OK on the return trip. With Argo I slept like a baby. Sailing in Galápagos was no problem for me. I didn’t even take sea sickness medicine then.