Design: depends on taste, but solid build on both
Size: DX200 is bigger, longer, thick but mot too thick. X5iii is more compact, easier to carry for daily life
Weight: DX200 is heavier
OS: For now, FiiO win (don't afraid too much when you heard about x7) But DX200 may improve in later FW but for now it not as fluid as FiiO
Battery life: FiiO have longer battery life for 1-2 hours if you play the same format in both DAP
SQ: For short, x5iii can fight what cost double of its price but not DX200, DX200 also can fight what cost double of its price.
I owned x5iii, I own DX200
I love them both
You can entry FiiO, it sound great and won't disappoint you. You can save some money to upgrade your IEM. If you already own great iem, try to get DX200.