If you want to store erc20 tokens, then using my ether wallet in a offline computer and making offline transactions are more secure than this one. If you once get online then your phone is not offline anymore.
Thanks for your answer.
Sadly, I don't have erc20 tokens. My Altcoins are not connected to Ether and mainly stand-alone coins (for e.g. SYS is one of them). Otherwise I would go with my ether wallet ofc ;-)