The address style is different. ERC20 starts with the number 0 and lowercase number x. OMNI starts with the number 1 or 3. ERC20 uses the Ethereum network. OMNI uses the Bitcoin network. In addition, the network congestion is different. The Ethereum network is often congested while the Bitcoin network is occasional congestion.
The transfer speed is different. The ERC20 transfer speed is faster, generally within 10 minutes while the OMNI transfer speed is slow, generally within 2 hours.
Security is defferent, OMNI is more secure than ERC20.
Fees is different, OMNI has higher fees than ERC20.