Appearance
Human hair blends with your own hair and photographs naturally. Synthetic fibre has a more uniform shine, which reads well in bold colours but less so when you want an undetectable natural look.
Washing, heat and lifespan
Human hair can be shampooed and restyled with moderate heat, and can often be reused for a second install. Synthetic locs should not meet a flat iron or blow dryer, and are usually retired after one cycle.
Cost
Synthetic is the cheaper entry point. Human hair costs more upfront but the per-wear cost narrows because the hair can be reused.