Different centres of gravity
MyFitnessPal grew up as a food diary and has built its product around it. GYM RPG does workouts first and keeps nutrition simple: meal logging, and a nutrition streak that multiplies your workout XP and stat gains up to 1.5x.
Barcode scanner
In both apps the barcode scanner is behind the paid tier. It is not a reason to choose one over the other.
Using both
Plenty of lifters keep a food-specialist app and a separate workout app. If you only want one app, check whether GYM RPG's simpler food logging is enough for you.