In general, it's a less competitive match, but year to year it's dependent on the overall interest from applicants. The popularity fluctuates year to year, perhaps more so than any other residency.
The hard part is that there are very few places that are exceptionally strong in both fields. In many cases, you are sacrificing "great" training in one field. There are some locations that you do med/peds because you like medicine but the real draw is the pediatric training, and in other places, your pediatric training is just kind of tacked on and you get exceptional medicine training.
The med/peds residents I know are some of the most talented doctors I know. Most of them actually say that the pediatrics boards are more difficult than the medicine boards (which is confirmed when you look at the overall board pass rates for each specialty). Do you sacrifice somewhat from being the best internist by doing med/peds? Probably, but you're also not the greatest pediatrician you could be. You miss on electives that round out your education in both fields, but what do you expect from only doing 2 years instead of the normal 3 in both fields?
I think that if you're someone who wants do primary care but has no interest in any sort of OB, than Meds/Peds is by far the best choice. You'll end up being a competent internist and pediatrician. Very few FP programs will be able to match the level of ICU experience a med/peds resident recieves whether it's the NICU, PICU or MICU. And let's face it, the hard part of being a doctor is taking care of the really sick patients.
The other major strength for a med/peds residency is that it keeps your options open. You can go into any fellowship from medicine or peds, from Neonatology to Geriatrics, and you're uniquely positioned to be the specialist that transfers care from pediatric concerns to adult concerns. This is a niche that is growing exponentially as the pediatric cardiologists and pulmonologists are getting better and better at keeping their patients alive to adulthood, while the adult specialists have no experience in the pediatric/congenital disease processes like congenital heart disease or cystic fibrosis.