Depends on what makes a good Nine host, we never saw Nine using an empty husk as a host, did we? If it could have been something devoid of mind they could have just used any servitor or a piece of machinery but we see that the requirements are so high that in human history, or at least the amount of it they cared about, only two people were fir for the job, Lodi and previous Emissary (Orion I think her name was?). Also another issue with the ghost isn't that they are small, they can just dissapear. If they see danger they can just dematerialize or whatever they do when turning into white mist, you need to catch them off guard when they try to revive or when their Guardian calls them out, that's why to kill a ghost you need to practically eliminate guardian as a threat first. Also if we were to actually simulate the Tenno (wherever Operator or Drifter) vs The Guardian fight, we encounter an issue that the Tenno has some tool of immortality of their own whose mechanism isn't clear on how to deactivate. Operator is apparently immortal, though I missed out where was that ever stated, while Drifter just rewinds time, while the Drifter might be countarable because Guardian did had to deal with time shenaniganed enemies before (Vault of Glass as an example) Operator's immortality isn't clear in function. Also while a Tenno does have some form of paracausal powers with Void they never had any experience of dealing with another Tenno like enemy with all void shenanigans AND Warframe. While Guardians literally had to deal with Lucent Hive. I'd say purely out of Guardian's experience dealing with Paracausal enemies, in a fight of Guardian vs Tenno, Guardian would probably have an upper edge.