Since you obviously have thought a lot about this, I hate to bring up a very important "if" about the Greece segment. You've chosen the Most crowded, Most Expensive, Most Promoted and Most "tourisized" Greek islands out of the 100+ available to you. Furthermore, your dates occur, unfortunately, on the MOST crowded jampacked week of the Entire Summer. Whew!!!
The Biggest holiday week of the year in Greece centers around Aug. 15, the Dormition of the VIrgin, every Greek who can possibly do so will take off the week that includes Aug. 15. Therefore both islands will be crammed to the MAX, Greek tourists piling on top international tourists; pathways of Fira and Oia - the 2 main Santorini villages on the famous View - will be like Times Square at Rush Hour in NYC (except in NY is about 32°C in Aug. Vs. GReece about 39°C). PLUS weekdays M-Th are busiest for Mega-Size cruise ships. About 3 daily, 2500-3,000+ passengers each, adding more than 9,000 visitors to main villages between 9-4:30. Finally, if you pursue this plan despite cautions, you may find that already most of the desirable Santo/Myk hotels in your budget range already will be fully booked.
I understand that your isle visits are in search of nightlife, which both islands offer, and beaches (which Santorini does not). Would you possibly consider another island? I'm thinking of one that would be more do-able: great beaches and good High Season nightlife, more room & MUCH less of a mob scene --AND the opportunity to "have your cake and eat it too" via DAY EXCURSIONS that enable u to SEE 1 or both of your 2 top-choice islands?
I'm suggesting PAROS, a wonderful island, fab beaches/views, but NOT the huge mob influx because airport takes only small planes & port cannot take giant cruise ships (hurrah!). There are delightful honeymoon places to stay, w pools, views, all amenities. PLUS, 2x a week, Day excursions go to Santorini, Ditto to Mykonos!! (2 excursion vessels, Naxos Star & M/V Cat Alexander - we recommend latter; it's larger = smoother rides & Aegean can be ROUGH in August!). http://parosbooking.com/index.php?option=com_k2&view=itemlist&task=category&id=17:one-day-cruises-paros&Itemid=86
I would especially recommend this for Aug 14th - 17th You listed for Santorini. Stay in Paros, and TUES take a day-trip to Santorini... then come back by 6pm for the nightlife in Paros. You can also day-trip to Mykonos, but if you really want to do the Mykonos nghtlife, you could transfer to Mykonos as scheduled (IF that is, u can find a booking).
The strategy I would recommend -- if you choose the Paros Option -- would be to book a Paros place for the full 8 days (many hotels have a policy of cancellation/date change without penalty if adequate notice), then try your luck with a Mykonos hotel search for 4 days. If you find what you want, you can shorten your Paros hotel stay accordingly.
At the time you're going, I suggest something around Naoussa the village at the top of the island .. it is where excursions leave from, or nearby, it has charming nightlife & fine restaurants, and not as bustly as Parikia the port town. The kind of place with pool & amenties u might like might be Hotel Petres, Hotel Stella Mare (right on a beach), and Kosmitis Hotel (also right on a beach, 10 min walk to Naoussa center).
If this interests you, here's the official island URL http://www.parosweb.gr and here's a gi-NORmous website with photos, maps, events, everything --http://www.greekhotel.com/cyclades/paros/paros-photos/home.htm