I can't think of any knowledgeable person who I think would claim that Plutarch's Sparta is more accurate than Cartledge's Sparta. Whether Cartledge inspires or titilates the reader as much as Plutarch does is another question, but not the most important one (though I'm not saying it's not important at all). Plutarch takes an extreme point of view on the question/problem of Spartan austerity, and it is one that must be counterbalanced by modern historians' and archaeologists' combined work on the subject. Only a view that takes into account their findings (which range from understanding that Lakonian artistic production did not decline sharply at the end of the archaic period, that foreign currency was not prohibited as some ancient authors claim, that there was less public, equal landholdings and more private ones than has traditionally been presented as the case, etc.) and is more critical than just accepting Plutarch at face value or only in conjunction with the Spartan Constitution, will yield an accurate view. I've personally presented papers on the exaggeration by later authors of events that occurred in seventh century Sparta (the "banishment" of Archilochus). Plutarch (2nd century CE) says Archilochus was turned away from the city gates because of his poetry. That sounds great to Plutarch, fits his need for morally uplifting and corrective stories, and it's terrific for Roman audiences too. But the problem is Archilochus wouldn't have been turned away in the seventh century - even Valerius Maximus in the 1st century CE says it was his books that was turned away (later), not the poet, and he is our earliest source. Not a really big deal, but just an example of Plutarch adapting stories to fit his needs and 'the big picture' better.

Plutarch would have been a big fan of 'truthiness' actually. It feels right, and fits with what we think about these people, so that's more accurate than whatever these mumbly-jumbly historians of today can add to the story or our knowledge! But the problem is that Holt's Baktria is more accurate (if not as fun for some folks) than Tarn's. Cartledge's Sparta is more accurate (if not as "truthy") than Plutarch's. Doesn't mean C. doesn't use Plutarch as evidence though, just that it takes more than that sort of extreme view to get the proper picture. Thankfully Cartledge and Holt, as examples, don't just write for stuffy ivory-towered classicists though - their work is much more accessible and has reached many more people thankfully.

The weird thing is, among most classicists I know, I'm the one that is always taking a position moderately closer to yours. I found a lot of interesting points and a lot I agreed with in Who Killed Homer?, but I also realize it really only applied to a few of the most extreme professors I've had too (Hanson's "Mandarins"). A couple of my previous professors and now one of my current colleagues have pretty radical views on many of these matters - but they're not producing things that are important either. The ones that were really good had very wide ranging knowledge, and the best compiled a definitive work on a Hellenistic Anatolian deity. The kind of thing that might bore you, but that is rock solid in the evidence and view of the god that it presented at the end - and it was one that only existed in fragments prior. I'm just saying don't throw the baby out with the bath water (like some of those professors do with Plutarch, and like you do with all modern historians or classicists).