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.