We stayed onsite for the last 3 years counting this year. This is a must if you want a relaxing time at the parks. The express will entitle you to 1 ride on each attraction but at $50 each approx i would book a room as suggested, for 1 night, check in early from 7am they will give you room keys which you show at the express line, room will probably not be ready, and the next day that you checkout you can use the room keys up to park closes, and you can ride as many times as you like using the express line.
Check the prices of express for 3 at each park and decide for yourself, and if you decide to book a room for 1 night try to see what offers they have i.e Triple A rates , supersaver rates anything that reduces the price. The dates you are going on, in August last year after the 14th August prices came down as season changed think rates where about $169 +tax this was tripleA rate which you can book if you are a member of the AA here