Funny I just made that decision! I chose a one day Park to Park with Express passes. We haven't decided if we are staying in Cabana Bay and buying our Express passes (which will be $99 a person), or staying at one of the other 3 resort hotels and using the built in express pass. The price isn't much higher at all for the other three hotels once you add in express passes. The best answer to your question does depend on where you stay. You could stay one night at one of the 3 on sites and get 2 days of express pass out of it (they are good on check in and check out day.
Note, we are a family of six age 9-44, so we don't have a huge a range as you guys, which means there are LESS things that we will choose to do (for example we won't be doing any kiddie rides; we mostly like coasters and Harry Potter, we don't love shows). If you want to do it all at both parks, you might be better off with two days. You could always try Express Pass for the first day and use that to scope out your plan for the second day!