I would say challengeri, I've seen few challengeri and I find differentiating spectabilis and rosei a pain but I've certainly never found upper labials to be at all helpful, below is a link to a rosei showing 6 upper labials, one less than the usual 7 putting it in direct overlap with spectabilis and challengeri. The head characteristic is more interesting, I've never noticed much difference in that and would be interested in trying to use it next time I'm IDing these.
The reason I don't go with rosei is similar to Bushmans, rosei may be highly variable but almost all of its variations include more pattern than other species of saproscincus, whereas this individual looks extremely plain. I don't believe whatever its validity that head length/shape can be used accurately in an image where the individuals head is angled strongly away from the camera as it is.
Those white dots on the tail may not rule out anything, but they are certainly prominently present on every challengeri that I have seen. Form one of the key features highlighted in wilson and swan for the species. And while as I say they don't rule it out, I have never seen those white spots on the tail of a Sapro that I have ID'd as rosei.
To clarify what method I use in the field for ID, I usually use supracilliary scales and found enough skinks to have at least some individuals for whom the counts are not in the overlap zone between species.
I admit that this particular genus is to me one of my greatest challenges in ID, since apart from the supracilliaries I've never seen a characteristic that I can use to get anywhere near a positive ID.
I've also disagreed with Scott before, and like you guys, usually come off the worse. But safety in numbers?
All sizes | Saproscincus rosei | Flickr - Photo Sharing!