Fords or Holdens?
At the end of the day they both tell you the information you need to know, cost about the same, and it's really not that hard to switch between the two of them. Don't forget that if you go down the Airservices path you'll need to buy TACs and ERCs as well.
The cost saving with the DAPs really only comes if you need a specific area (e.g. working in WA and only need DAPs West, an ERC and one or two TAC Charts). From memory Jepps works out cheaper if you need all of Australia (ie it completely replaces your AIP, DAPs East and West, all TAC and ERC charts).
It really is personal preference when it comes to the IREX.