Do you really think that adcoms are going to care which one of these you pick? If one of the projects sounds more interesting to you, then obviously go with that one. If both seem equally interesting, then go with the one that you think will give you more responsibilities and the one that will give you a better overall experience. If you have not already done so, you should ask the PIs (or whoever you've been in contact with) what sort of projects you may be involved in, and what sort of work other undergraduates have done in the past. This should give you an idea of what will be expected of you, and can help determine which lab will give you a better experience. At the end of the day, the "personal growth" you obtain from this research will be important to adcoms, not the field that you did your research in (especially when your two choics are fairly similar).
This.
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I don't think adcoms will care to any significant degree, or perhaps even at all, about what particular type of biological research you conducted. They may care that it's biology (though I've heard this idea questions as well), but beyond that, it's all good.
So go with whichever lab feels like it would be more fun, more interesting, and more exciting. Honestly, if one PI seems like they would be better to work with than the other (nicer, more understanding, easier to get along with in general, etc), then maybe factor that in as well.
Finally, not to be a buzz-kill, but I recommend you don't go into this with your focus on being published. It's possible to work in a lab for a pretty decent amount of time and never obtain a publication. Science moves at science's pace, and unless you join a lab that's publishing nonstop for whatever reason with or without you, then it's not a good thing to get your hopes on.
Besides, there are dozens of benefits to doing research, other than getting published. Enjoy research for its own sake. In an idealistic sense, you will be helping to give humanity new knowledge! And in a practical sense, a lot of lab science is just really, really cool.
Good luck!
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