Erma Bombeck was a thing way back when. A housewife-humorist with a column about suburban life. When she strongly supported the Equal Rights Amendment in the 70s, a lot of more conservative papers pulled her column. She was also a breast cancer survivor, but didn't talk about it. Here is a quote of hers I came across about "spouse shopping"
People shop for a bathing suit with more care than they do a husband or wife. The rules are the same. Look for something you'll feel comfortable wearing. Allow for room to grow.
Leaving aside the problems with the concept of "spouse shopping", or rather using shopping as a metaphor for choosing, I think the same applies to finding a thesis or postdoc mentor. People, students or trainees, are concerned with the specific topic and whether they are going to work on a particular problem that has caught their interest. There is nothing wrong with using that as a first criterion for narrowing the field.
But far more important are a host of other factors that have nothing to do with the actual science being done in the lab.
What is the reputation of the mentor? How do they treat their trainees? How often does the mentor talk with the trainees? How much is the mentor around to talk to trainees. Does the mentor's style match yours (do you need to talk with them all the time, would you rather be left alone to work out problems, how do you feel about micro-management)? You can find these things about by talking to the trainees. You can ask some pointed questions along these lines. You can also ask more subtle questions: what is the turn-over in the lab? How long did the last phd student/undergrad/post doc last? Will the mentor ask for the current trainees view of applicants? How does authorship work in this lab?
Ask yourself: am I going to learn something new, even if I'm going to Lower Podunk University instead of The University of Big Swinging Genitalia? Will a project I do in this lab make me more employable, make me understand science better, help me work on those bigger things that interest me?
In short: can you see yourself being comfortable in this lab? Is there room for you to grow?