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I am applying to biology REUs(Research Experience for Undergraduates) this summer. Would a white female junior from a small R2 university, 3.9 gpa, and one summer of research experience be likely enough to get into one these programs? I've heard that it's a long shot if you're not a minority, the issue is while female counts as minority for other REUs I don't know if it counts much for bio these days...
I'm asking because there's a very very good study abroad program (a very good university to study at for bio) I want to apply to that goes until June (which overlaps with 99% of REUs and other internships). So I either pick the top study abroad program and simply do research at my university over the summer or get a job, or I pick my second choice study abroad program in order to apply to REUs (as well as other internships such as NOAA), but risk not getting into any of them anyway and going right to my backup plans for the summer.

I don't know where you are getting the idea that it is a longshot specifically for non-minorities. In general, REUs do have low acceptance rates for everyone. There are a lot of students who want to do them and not that many REUs.

Now, to answer your question: yes, being female will likely help somewhat, but it likely won't make a big difference. What will matter more is the GPA which will help out a fair bit. The previous research experience may hurt or help; at least in math some REUs try to admit undergrads without experience to spread them around, but that's not all of them by any means, and for the most part it would be helpful. I suspect the situation in biology is similar.

Since you are a junior, you should already have some connections within the biology department, either your adviser or faculty whose classes you have taken. They can likely give you more personal advice since they know you better. But at least from what you've said to us, I suspect you have a higher chance of getting admitted to an REU than the average candidate.

Unfortunately there is no meaningful way to estimate your chances of being accepted into any specific program or set of programs (REU or otherwise), because selection is not strictly random, nor is the criteria uniform or transparently available to applicants.

However, while REUs often mention the goal of increasing participation of minority candidates, in practice this varies from "a strict rule where only minority candidates are chosen" at some few programs to "its just something we are supposed to say, and the vast majority of people who end up in the program are not classifiable as a minority but that's not something we make a point to mention on our website" at others.

I understand the anxiety these sort of selection decisions cause, especially given the tremendous ambiguity and opacity of decision criteria, and especially the fear that vague bureaucratic-double-speak can cause.

My advice, having done an REU myself - and to even apply I had to push past a lot of fear of wasting my time and thinking I had no chance of being selected for any, but then being offered multiple options - is to apply if it is something you might like to do, and that's it. You cannot count on being chosen, or even hearing back on a decision from all places you apply (I had at least 3 big-name programs not even notify me of selection and they ignored my email follow-ups to their program coordinator).

If you have a good option that requires material commitment (like, say, a big deposit or signing some contract with explicit big penalties if you change your mind) before knowing what the other offers would be, I'd encourage you to decide what your taste for risk of it not working out is - i.e., with big risk comes the need for a good backup plan.

Personally, I had to roll the dice between summer employment and REUs that had very different decision timelines. I ultimately realized many of them had more flexibility than I realized, and while I would have preferred to not give late notice on turning down an internship and another REU offer I was otherwise excited to do, it worked out and people were far more understanding than I would have expected they'd be. You are a student, and these are opportunities meant to help you - so most places aren't going to be heart-broken if you have to change your mind, etc.

See what sort of flexibility you have on changing your mind if some options turn up you didn't expect, and then if there is an option I always suggest you roll those low-price application costs and see what happens.