Given that extra terrestrials are a near certainty , and no evidence for the later presents, I'd say the former.
both have equally logical tendancies to exist. Now, proving that they exist... there we run into... issues.
Great question. Extra terrestrial, since any that are extra dimensional are also extra terrestrial, so even if each extra dimension also has an earth there would only need to be two extra terrestrial races in any dimension to make more than the earth - or something like that! :-)