It seems like it should be ”saber alter,” since "artoria pendragon alter" is actually the same as "artoria pendragon alter rider" in e-hentai (Tag Definition). In the other hand, "artoria pendragon" might be "saber" either. Actually, the author says that they are "saber" and "saber alter."
As I said before, the tag "artoria pendragon" is not the same as "saber" in e-hentai's tag definition, though they are technically the same person. The former is assigned to the elder one (the lancer version).
For a similar reason, "artoria pendragon alter" is assigned to the Alter version of "artoria pendragon," but not to the Alter version of "saber," aka "saber alter."
More information can be found by "Show Tag Definition," and its discussion page.