Well - what do YOU want to do?
I chose PM&R Sports pathway over FM pathway because I truly only enjoyed the MSK, neuro, neuromuscular, and cerebral palsy pathologies going into residency. I also felt like PM&R prepared people much more thoroughly for understanding MSK, biomechanics, gait, spine care and thus appropriateness for identifying pain generators and thus providing appropriate intervention as a MSK focused physician.
PM&R sports physicians can be/are weaker in the medical management of sports teams (vaccinations, diabetes management, cardiac screening, etc.) than FM or IM counterparts - but I don't feel terribly substandard compared to them and if I don't know something I can always research it or send it to a cardiologist, endocrine, etc. if I feel it is truly dangerous or prohibitive to competition.