Capn Bloggs,
Nothing in particular! Fly whichever ILS you like; they are both exactly the same (the DME used is irrelevant). The initial replies from ATC indicated that. I suspect that he finally caved in and gave you an answer that wanted to hear.
A question though- In many of the ILSs overseas the difference in the Y,Z etc lies in the missed approach path. I am not sure if this is the case in Syd.
Would it not be prudent for ATC to specify the letter suffix to ensure the correct missed approach is flown?