I had to make the same decision last year. I'd be interested in what others think.
My opinion for FP over Peds:
1) Continuity of care into adolescence and adulthood- My outpatient pedi preceptor was uncomfortable dealing with sexuality, gynecology, substance abuse, and pregnancy, depression and anxiety. (Basically common "adult" issues that kids deal with). Economically, you're more diversified in terms of your patient/revenue base. There's an oversupply of pediatricians and undersupply of geriatricians. FP allows you to take both populations on.
2) Comfort with non-pedi-internal-med issues because they're built into the curriculum: Most FP curriculum have Ortho, Sports Med, Neuro, Derm, Uro, and ENT built into them. Programs vary but these specialties see both adults and kids. Of course, for FP you can extend your training using elective time. Most Pedi curriculum have these too, but they're mostly structured as selectives/electives.
My opinion for Pedi over FP:
1) Comfort with the sick kid. You see more kids and sicker kids in Pedi than in FP. By volume alone, you should see a lot of pathology repeatedly. There is much more hospital care, ICU care, and high tech Neo exposure in the Pedi curriculum than FPs. You'll find that this gap is wider in the unopposed community FP programs vs. university FP programs. The really sick kids with complex pathologies needing complex procedures/research protocols usually end up in university setting.
2) Economically, Peds is better positioned to see kids than FPs are in seeing kids. See:
http://www.aafp.org/fpm/20050700/45cari.html
All of these variables can be changed by taking on electives and tailoring your practice to fill in gaps. The real world question usually is how "comfortable" are you (how much liability are you willing to take)?