Switching research areas & PI expectations I'm an undergraduate, 3/5 years. I've worked in the same basic-genetics lab for three years now (mostly part time, 6 months of full-time).

I'm hoping to apply for MD/PhD programs doing computational biology, and I have a little bit of experience (an internship and some part-time industry work) with translational computational but not much.

My PI, and my most supportive mentor &amp; letter-writer wants me to work another 6-month full-time co-op in his lab. 

On one hand, I am very interested in the research we're doing. The 6-months would be a chance for my first not-nth-author publication and could be fun. And my PI seems genuinely disappointed/hurt when I do research elsewhere, which is actually a big influence.

But I don't want a lack of relevant experience to hinder my graduate school application. I could use that 6-month period to work with a computational lab or a hospital or industry.

I can't be the only one to be in a position similar to this. How do I even start to make a decision?
My two cents based on this part of your question:


  On one hand, I am very interested in the research we're doing. The 6-months would be a chance for my first not-nth-author publication and could be fun. And my PI seems genuinely disappointed/hurt when I do research elsewhere, which is actually a big influence.
  
  But I don't want a lack of relevant experience to hinder my graduate school application. I could use that 6-month period to work with a computational lab or a hospital or industry.


You need to decide which option is most valuable to achieve your career goals between a published paper in your current field (be aware that staying 6 more months might not be enough to get the article published) and some experience in a complementary field. You must absolutely refrain from basing your decision on your PI's emotions: of course, all PIs want to keep good lab members as long as possible (and it is flattering to get such recognition), but this is about your life and career, not your PI's, and if you think it is time to move on, that is your decision.

If you're concerned that leaving would be detrimental to your professional relationship with this PI, then you should think about ways to tell them your decision in a professional and non-emotional way. If they cannot accept your rational decision without drama, it is a good indication that you should not spend more time working with this person.