Well, first off I'd say you definitely don't want to post anything comparing two (or more) books asking which one is "more" authoritative; that's just likely to attract argument and debate between proponents of each book rather than any actually useful comparisons.

That said, I think the crux of what will make your question fail or succeed is: **What do you mean by "authoritative"?**

I've not read any of the siyar you've brought up, but as Medi1Saif had mentioned there's points of contention in all of them, mostly related to the sources used to generate the content. This is to the best of my knowledge a common trait of siyar, since even if you can take *all* the sahih hadith and string them into a coherent chronology, that accounts for a fraction of the prophet's entire life and ministry: If one wants to fill in all the gaps, one must resort to less reliable sources, and how "less reliable" that gets depends on the author.

However, is this really a problem, or even relevant to what you're *actually* asking? Ensuring that hadith are authentic is *essential* for fiqhi matters in pretty much all schools of jurisprudence, but **one does not derive fiqh from a sira**.

So setting that aside, what is, or is not, "authoritative"?

Now I'm just speculating here, but I suspect what you're *actually* interested is less whether they are or are not widely-regarded, rather **whether there's any significant reason to *disregard* them**. In that case, I would define "authoritative" as "possible, within reason" (which would probably get me shot by a dictionary editor). In other words, it…

  1. … is written by a **scholar with sufficient credentials** to know what he's talking about
  2. … is comprised of information that **doesn't contradict known authentic hadith**
  3. … contains **no stories that are outright *fabricated*** (as compared to weak or disputed)

And a bit more subjective, but I'll throw this in for completion,

  4. … is **consistent with well-established Islamic beliefs**

Point 1 may warrant its own question for each book, but I reckon between Wikipedia and the forewards of the books themselves, you're probably already covered about as well (if not better) than anything you can get here.

Point 2 can be reasonably scoped by requesting *explicit* and *concrete* examples of *clear* contradiction. If the contradiction is based on *interpretation* rather than actual *text*, it should probably be rejected unless it's clearly established how *common* that interpretation is: Even a fringe interpretation would probably be a valuable point of consideration for your purposes, but it is important to distinguish that from one that's widely-held among most/all Islamic scholars.

Point 3 is similar; what is or is not an outright *fabrication* is not clearly defined and can be a point of dispute among scholars, but so long as the answers clearly establish how *common* that interpretation is it should be safe from opinion-bait. Ideally, this would include concrete references to reputable scholars, since appealing to *personal* opinions of what is or is not fabricated is just…messy.

Point 4 is a lot trickier: I included it for the sake of completion, but making it work I'd need to leave as an exercise to the reader. Personally, I'd probably avoid including it since it's hard to make it *not* be opinion-bait: There's so many potential variations (subtle and major) of Islamic beliefs when you work under the umbrella of Islam as it's defined on this site (i.e. everyone who self-identifies as Muslim), and if you distribute those over the entire lifetime of the prophet you're bound to find innumerable points of contention.
  
So if I were trying to ask your question, I'd probably scope it to points 2 and 3: Something along the lines of "**Are there any reasonable criticisms to the veracity of [book name]?**" (except maybe with less clunky wording…) with the question body emphasising what counts as a "reasonable criticism", namely that you already realize that they probably contain *weak* narrations, and are particularly interested in **concrete examples of actual conflicts and fabrications**.