I've stayed at both as a Platinum - at the Holiday Inn I've been upgraded to a suite, at the Crowne Plaza I was upgraded to the Executive Floor. My stays in Melbourne are generally across weekends, so I think that helps (so long as there's no events on over the weekend anyway).

In the end it comes down to what's more important to you, extra space or a nicer view - I don't recall hearing anyone being upgraded to a suite at the CP, whereas at the HI I'm not the only one that has had a suite upgrade (they have a fair few suites there). At the CP the rooms aren't particuarly large, and there's not much in the way of additional benefits of the Executive floor, so it depends on how much a view of the Yarra River thrills you I guess. . The rooms at the CP are a bit nicer, but there's not a huge difference in quality between the two, certainly not given the premium the CP often seems to charge these days.