I really would suggest staying onsite for one night.
Keep in mind, with a ONE night hotel stay onsite, you get TWO FULL DAYS of unlimited express!
If you were to purchase Universal Express tickets, they would only be good for one day. They would also only allow you to skip the line of each ride, once! With the hotel key, you can ride the same ride over and over again in the same day.
For Thursday and Friday, purchasing an express pass would cost $31.99 per person, per day. So your total for 5 people for two days of express would be
$319.90. As you can see, staying onsite is CHEAPER than purchasing the express pass at the park (which is normally always the case for groups of 4 or more). So you get to stay at an amazing onsite hotel (some say they are better than Disney Deluxe, and I have to agree), get to take advantage of the early entry to the parks on friday morning, get to use the water taxis to the parks, save your DVC points, and of course skip the lines!!
Just make sure to wake up really early on Friday, check out of the hotel (BUT KEEP YOUR ROOM KEYS!!!!), and pack your car before heading to the parks for the day. It is not fun to have to return to the hotel after your early access to HP to check out and pack the car, only to turn around and head back to the parks after. Get it done before your day even starts.