Good question Saz, but before I answer, a word of advice ---
When on Forums asking for good free advice from travel vets, it's thoughtful to give all key facts, not make them ask you for them ... i.e., actual dates (which turn out to be mid/late May for Greece, not April) ... and another Must: your budget in EUROS, no your currency. Don't require your helpers to do the math. http://www.xe.com/currencyconverter/ is an INSTANT conversion site -- and now I know that $150AUD = 98 Euros. Wow! you are lucky! I've been to Greece 12 x since '99 and when sharing a room easily manage on under €60 per day for EVERYTHING except splurges: lodging, meals, transport (ferries etc) entry fees, miscellaneous, and even a sundown glass of wine and 1/2 liter each of house wine at dinner. (NOTE: typical "taverna" dinners in Greece can range from €8 - €17 per person for a hefty main-dish plate w. potatoes & veg + 1/2 liter house red per per person (add €2-3 ea for a shared greek salad or major starter).
After France& Spain and then an organized tour (which always cost you more) you will be THRILLED at Greece prices -- it's the MOST budget-friendly country in Europe. Of course Athens cost more, like all Capitals, but if you can't book ATH Backpackers, another FINE hostel is ATHENSTYLE - http://athenstyle.com/en_GB/ - just 200 yards from Monastiraki Metro station (the line for Piraeus). In May budgeteers can find a double r in Santorini ON t he caldera side for under €40, a double right on beach at other islands for €30-40, picnic at lunch for €4 usually ... and while a day-trip to Mykonos could cost €55. a LOT less than staying there! You'll have a good amount left for beer & wine (watch out for $$$ hard stuff).
Now to question: Paros vs. Ios?? Istill say Paros, & Reason I say this is Parikia has good club nightlife in Parikia (in May I hear it going on, on my way back to hotel, around 11), see lots of 18-30 people around. and ASIDE from partying & beach, Paros has lot more to offer. Stay near edge of Parikia; tons of budget places walking distance from ferry going N are on OK beach (Livadia) & you can bus/ATV to the rest. IOS is mainly beach + party which doesn't really rev up until colleges let out. YOu mentioned Milos -- GREAT Island, and has fab round-island sails, but u do need car or ATV, local bus service v sparse & does not go to some of the great beaches.
Now MY question -- to YOU: This is a MAJOR trip costing $$$$$ AUD -- and you seem to want to get ALL info from Internet. News flash: NO CAN DO. These FOrum threads can bring advice from people who've had ONE trip and share very limited experience (i.Santorini FAB, Athens SUCKS etc). And travel websites are often just hypes, not candid about ANY drawbacks. NOTE: one exception is http://www.greekisland.co.uk -- tells both the good AND less-good aspects of many many islands.
Solution? Surely you can spare €15-20 for a decent CANDID guide aimed at your age group: try the Let's Go books. Also, my fave overall (& I have 2 shelfs-full) is ROUGH GUIDE to GREECE. No need for big $$ outlay -- Australia has good libraries. go there & browse guidebooks ... photocopy what u need. And/or use Amazon or other similar and order USED copies online (no need to get latest; anything back to 2010 is fine -- you're not seeking 2016 restaurant prices). When you read some CANDID & knowledgeable stuff you'll be able to judge forum advice better. Your comments so far indicate you're relying a lot on "I hear that" but you have no research to be able to assess the value of that hearsay. You've got plenty of TIME... so use it, to fact-find and share your ideas... the trip will be that much more enjoyable. Happy planning!!