If you stay onsite at Universal, you get express passes for your day of checkin and your day of checkout. So the cost of one night gets you express pass benefits for two days.
We are splitting between Uni and WDW on our next trip, and are staying onsite at Uni for our first few nights. Adding up the benefits of the express pass lines, early entry into at least one of the parks for HP stuff, and ease of getting to the parks from hotel for an early morning, it just made more sense for us.
We are planning a car service from MCO to Royal Pacific at Uni, will likely take a cab or car service over to WDW, and will take DME to the airport for free. In all, costs of rental cars were going to be north of $350 for the week, and two car services or taxis are less than half that. Plus no costs to park at Uni.
The child and I don't do well in the heat, so being able to bypass a lot of the line-standing with the express passes was a big deal. We had also considered coming over from WDW during the day, but being able to take a mid-day pool break at the hotel was also a significant consideration for us.
Plus, we found that the deluxe level of Uni hotel was still more affordable than a Disney deluxe hotel. And, we were easily able to use points and rewards to book at Uni. We got a free night with Amex points, and I was actually able to get a second night at more than 50% off with my hotels.com free night redemption (I booked two reservations at Royal Pacific and contacted them to link the reservations, they are very nice and will try to keep us in the same room). I feel like there were multiple options to apply points and rewards to the Uni stay that maybe aren't available for Disney? At any rate, the Uni stay onsite ended up being very reasonable, and it looks like a very nice hotel and nice pool.