On the days people do Universal, do you tend to leave the Universal park with time to do an ADR at Disney restaurant?
NO! traffic between the two parks at rush hour is often slow. Not worth doing unless you are constrained by the
DDP.
is it best to eat at Universal or just leave the night "open"? Thanks!
YES! Both!
One beautiful aspect of Universal is that you can get decent quality food (almost) whenever you want! Most places, most times, waits are under 30 minutes. Really! If you want a reservation, you can make them 3-24 hours in advance to avoid any wait. I do suggest making one for Mythos.
The other places that get a little busy are the Harry Potter QS's, especially Three Broomsticks during peak lunch on busy days. Mostly, we've had our food in 15min. The food and theming are SUPER though!
Universal offers many options at all different price points. Overall we find the US food to be better than WDW. Universal has the new free choice Coke machines. Super for free water refills! It blows my mind that WDW doesn't have them.
There are also many offerings just off property. Next door is a plaza with a fun Whole Foods. Not inexpensive, but it has an extensive hot bar/salad bar: over a dozen soups, bbq, fresh prepped seafood+sushi, full bakery, fresh squeezed juice bar, brick oven pizza, etc. Some I-Drive places are to be avoided. Though if you happen to like ANY American chains, there's probably a branch in the WDW/US vicinity!
We've only tried a few options in the hotels, but we were very pleased. RP serves sushi at 5pm. It was slightly $$ (expected at a hotel), but high quality, good portions, and NO ADR. Portofino's Trattoria has an all you can eat custom pasta option with meats/veggies, soup, even gluten free pasta choice. I think $26 adults/$12 kids w/non-alcohol beverage. (I'm not 100% on that price.)
Universal also has many adult beverage paired with food. options. In the park, we really liked the food at Finnegan's Pub. Some of the in-park QS options are fried junk, but much US food is now better than WDW.