You can hold your ERC-20 Agoras tokens in Metamask.
DO NOT SEND YOUR OMNI AGORAS TO METAMASK
You can hold your Omni Agoras tokens in your omni wallet, or in any Bitcoin address to which you possess the private key such as a paper wallet.
DO NOT SEND YOUR ERC-20 AGORAS TO A BITCOIN ADDRESS
If you use the latter method and wish to send Agoras out, you'll first have to import your private key to Omni Wallet