1) Don't rule out med/peds
2) If you really feel you have to do one or the other, I think it would be wise to do Sub-I's in both. That's really going to give you the clearest picture of what life is like as a resident in each field. Certainly it's extra time spent doing difficult months, but we're talking about figuring out what you want to do for the rest of your career.
3) It might be possible to do a month of Family med or a combined med/peds clinic, and see which group of patients you get more excited for. May help, but I think sub-I's are going to be higher yield.
4) You probably do have a preference, or perhaps you're not sure if you can really give up one aspect you love about one or the other. I know for me, I loved the complexity of inpatient IM and juggling multiple diagnoses - even if they were in for pneumonia, you had to manage their diabetes and their blood pressure and whatever else. But I was able to find that same sort of complexity in the PICU and so with Peds Critical Care as my career goal, I was fine leaving the adults behind. There's lots of overlap like that, so you can probably find what you love in either.