When facing a decision like this, I think you really need to think about what there is to potentially gain from the experience.
If, for example, you were happy with your REU and got a lot of out of it and know that one of those fields is the one you want to go into, and you would just be taking on this research project because you felt you had to, it might not be the best idea.
On the other hand, if you're still exploring different areas, another project might be a good idea.
Doing research in the summer between finishing undergrad and graduate school is great, but it's going to be too late to help you get into graduate school if that's the plan. You would have already been accepted by that point - unless you're planning to take a year off. That said, I've observed that some of the most successful researchers are the ones who take every opportunity they can to do research, not to get them to the next step, but because for them, doing research is what it's all about.
It might also be worth pointing out that for someone in that 3.4 - 3.5 GPA range, research experience is likely to play a big factor in any competition for graduate school positions. For many places that ballpark can be the cutoff for getting in or not and so having your name on a paper or an amazing reference letter can become a decisive factor with respect to whether you get in or not.