There is no equivalence in our two competing hypotheses. A SETI explanation for a hidden solar system base may be a long shot, but the speculation that it was erected by an older technology (say, 20th century) is completely untenable. Remember, the tech infrastructure required to do so does not exist now on earth, and it certainly didn't exist in even our most recent past. But even under the most charitable assumptions, even if we concede some person, corporation or nation was able to mobilize the tech for such a feat, it would have been impossible to conceal the massive organizational and administrative apparatus to implement it.

As for your second point, whether or not the location of the base would be Oort or Asteroid, that choice would be dictated by its purpose. If an alien civilization WANTED the site to be discovered in (its) future, (as in Clarke's Sentinel) either by the apes evolving on Terra, or some third party from out in the Galaxy, the Belt offers the best combination of a stable, safe, permanent platform and ease of discovery by a merely planetary spacefaring capability. On the other hand, if it was desirable for the station never to be be found, or to be discovered as late as possible, then placing it in the remote vastness of the Cloud would not only ensure its security, but allow it to continue its surveillance (and reporting!) activities with a minimum chance of being detected. The Oort Cloud, or even the Kuiper Belt, for that matter, is so huge and remote that even if we had the capability of FTL travel it would still take us centuries to examine every potential outpost there. "It only takes a moment to walk a stretch of beach, it takes an eternity to examine every grain of sand."

Still, I must confess, that SS babe in the poster (or the Sarah Palin look-alike in the campaign poster at left) might tempt me into changing my Party affiliation.