Perhaps this belongs under a post called "10 hard ways to fail a Ph.D."
An advisor-advisee mismatch can be excruciating.
For established mentors - Full or Associate Professors - if they aren't actively graduating students, there's something wrong. If they have current students who are taking, on average, 7 to 8 years, there's something wrong. If you want a tenure track position and this established mentor doesn't have a record of placing their students in those positions, there's something wrong. The concept of lineage really does help. Great mentors really do seem to birth new ones. If a particular mentor has a limited lineage, look elsewhere.
Otherwise, I actively consul folks to stay clear of nontenured faculty as primary mentors. As I indicate above, those folks have pressing priorities elsewhere. If you like someone who fits in that category, get a more senior mentor as your primary and add the younger one to your committee. Just make sure the primary is truly signing off on everything. The politics are always there, especially with a nontenured trying to prove themselves, but they are manageable with some savvy. The point is: Stay clear of the nontenured without a safety net. Not only are they distracted, but they have no standing to advocate and defend you, if necessary. You're easily expendable when the choice is you or their career.
Basically, mentors can be classified based on where they sit in the career trajectory and their history. If they don't have a clear and steady record of graduating students, then don't expect you'll be different.
That being said, I always tell other students to remember that their interests are most often orthogonal to that of their advisors and that ultimately they are their only advocates. Also, choosing an advisor needs to be assessed on a case by case basis.
I know of two criteria for tenure: Publications and grants. Student advising is not going to shift that calculus dramatically in any direction.
Nontenured faculty may be great but it's potentially too problematic to have one as a primary adviser. Too many unknowns affect their emotional and professional state to tie your ship to theirs. Better to get a more senior person as the primary mentor and then form collaborations with, and get advice from, the nontenured faculty.
For sure, it's a case-by-case consideration. A nontenured faculty with graduating students before they receive tenure is a good sign along with grants and publications. But a tenured faculty member is better equipped, at least politically, to look out for their students. If they don't have a record of doing so, stay far, far away.
My main point is that I don't think you can really say one is safer than the other. I certainly think early tenure track faculty have greatly divided attention and are under incredible amounts of pressure, which is not a good prescription for being a good advisor, but I think that a lot of what tenured faculty people do also can make them poor advisors.
As a student you will always be working with incomplete information, in particular, how you will handle a given advisor, even if everyone else says they are great (or horrible). Of course you have to simply play the hand that is dealt and make changes to your situation when appropriate.
On a side note, I was able to get one very senior person on my committee, who is very enthusiastic about my project. Even though he is not my direct advisor, he has been able to strongly advocate for me when necessary.