The practices of fellowship-trained PM&R sports docs typically differ from FP/IM/EM/Peds. They are trained in EMG/NCS, fluoroscopic and ultrasound guided interventional spine/pain procedures and do little to no primary care outside of neuro-musculoskeletal issues. One type of practice is definitely not "better" then the other....just different strokes for different folks.
Another major point to consider is the "worst case scenario"...ie you are not able to sub-specialize and must practice your base specialty. Which would you enjoy more on a day to day basis for the rest of your career...practicing in FP, Peds, EM or general PM&R?