Is it realistic to prefer a non-tenure-track research position?
Background: Answering this as a biomedical researcher currently in a tenure-track position who definitely looked at some non-tenure, 100% soft money positions. One thing to note, there are also effectively 100% soft money tenure-track positions out there in the world. What a tenured, all-soft money position actually means is left as an exercise to the reader.


  Is it realistic to hope to spend my career as a research professor, or
  is this usually something people only do short-term?


There are people I know who spend their careers like this, and some of them are happy. But there are also a lot of people who either leave for tenure-track positions, or switch over to tenure-track in time. But it is a career path - indeed, one of my former departments has an entire structured program for these people to make a career out of it, including wrestling with how do you evaluate them for promotion, etc. Of the people who have left, there were two major complaints in my experience:


Instability, in terms of having yearly contracts, no salary support, etc.
Many departments restrict non-tenure track faculty in some ways - most commonly, deciding on tenure, but also potentially on hiring committees, advising, etc. Some people resented feeling like 2nd class faculty.



  Can I have reasonable job stability doing this by working
  collaboratively?


Ironically, the most stable people I know in these positions are essentially "super-postdocs", and not actually on lots of grants, but well supported by a single, large center-type grant.

I will say that, as someone who works very collaboratively, and is also in a position where one of the most common tracks forward is building up a somewhat piecemeal funding portfolio instead of a single "one-and-done R01", I don't find it particularly stable, or at all less stressful.


  Do research professors tend to be more or less stressed than
  tenure-track professors?


Depends on the position. The ones I knew who were funded as part of a center, core, etc. where whether or not the application failed wasn't particularly on their shoulders? Less stressed.

The ones trying to scramble to keep 10x 10% effort projects alive and going at the same time, while worrying if they'd get their contract renewed? Very stressed, because you can never get off the treadmill.


  If you have experience with this career path (or know people who are
  in this career path) is it something that you would recommend to
  others? Why or why not?


I ended up taking a tenure-track job, and choosing one with a fairly generous amount of hard money, and don't regret this at all.

If you can find someone who is willing to put you on a large, stable grant that can support a substantial % of your salary, even if it is technically hard money, that's a decent path I've seen some people enjoy immensely. But if you're expected to find and bring in all your own money? You're going to spend as much time, if not more, scrambling for funding, without the carrot of tenure at the end.

It also depends on why the position is not tenure-track. In some places, that's just the way things are - for example, in one institution, PhDs basically couldn't get tenure, so their positions were non-tenure track, but their contracts were for long periods. In that case? Not a red flag in my mind. In another, it felt very much like those faculty members were very-low commitment bets on the university's behalf, and if they didn't work out, they were disposable. That's not a position I'd want to be in.