Mobile with Virgin mobile and Optus with prepaid modems

Hello my Wife and I have the internet service with Optus and the phone with Virgin mobile but at the moment we are looking to go post plan and put everything with Optus because they have a store in our City but there no store for Virgin mobile so we are very unhappy with the service at the moment with Virgin doing a few things wrong. The issue with Optus its a little expensive with a post plan of $30 dollars each months and they said with our prepaid 16gb we have on prepaid they have work out something toward this to a post plan deal, I tried calling them and I suggested can you do something like Vodaphone for 24 months on $22 dollars per month and Optus said no. So I'm thinking of a few situations, do I go on the Optus plan for 12 Months and then join Vodaphone afterwards while my Wife join Vodaphone now. Optus said I can sell the 16gb prepaid to someone else, so that a offer if anyone wants to take up or I go in store to optus and see if I can get help there instead? As again I only have 3 phone companies in my city which is Telstra, Optus and Vodaphone only. Any help or ideas is really appreciated. Thank you