I don't find either particularly formidable, but if forced to pick between the two I would say USSR rather than modern Russia+China, mostly because I don't see Russia being as connected with China as people say. I think they are buddies, but I am not so sure they would be allies in a hardcore direct face-to-face war with America.
I'd say I'd be more concerned with a war where we had to massively fight Europe again. So I guess by that logic I find Russia more formidable, though Russia has technically sided with us in our last two European wars. Either way if Russia were with us or against us, a European war is pretty much the most daunting for exactly the reason of it being a powder keg and how the alliances could fall out is not so certain and going by precedent it's always a pretty bad thing for everyone when Europe fights itself.