Applying MD/PhD for 2016, so grain of salt, etc.
The topic or field is less important than your ability to speak passionately about your research and show that you developed the ability to think like an independent researcher. If a psychology lab will open the opportunity for you to ask and answer your own questions just as easily as a biomedical science lab, then work in whichever you are more passionate about. This is especially true if one of the labs only offers undergrads two years of dish washing and fish feeding.
The other important piece from your research is the letter of recommendation. Assuming you give your recommender only good things to say, showing off your ability to think and act like a mature researcher will help you significantly more in the application process than washing the proverbial dishes in any field.
If you are worried about going into the PhD in another area and being lost around the new techniques and literature, look into summer research programs in different fields. In my case, I have done my undergrad work in a genetics lab, but spent a summer doing synthesis in a physical chemistry lab. You could easily do the same with any combination of bench work and psychology to get varied experience.
All that being said, Adcoms vary. Picking a lab near the field in which you want to end up will never be a bad thing, while working too far outside your PhD interest may be seen as a red flag that you aren't committed to your field. And if you want other opinions, check out the physician scientist board where questions like this crop up periodically.