You didn’t say what you’d do instead in that half day if you don’t go to
Epic. But, this is my take, as someone who did 1 day in Epic with an EP in early August: I'd probably go for the 1.5 days instead, for a few reasons:
A) For us, we ran into weather issues (lightning in the area) that closed most outdoor rides for the last 5 hours that the park was open. At Epic, that’s a LOT of the park out of commission (Donkey Kong, Yoshi, Fyre Drill, Wing Gliders, Dragon Racers, Curse of the Werewolf, Stardust Racers). In our last couple of hours there, our only options were Mario Kart, Monsters Unchained, the Carousel, and meeting Toothless (Ministry closed early that day, and the last of the shows were earlier in the day). You never know what weather will do; we got especially unlucky, I think, but we missed a lot of rides that we had thought we would go on that day. (We still enjoyed it, though!) Having 2 days means you can work around weather better.
B) The day we went, Battle at the Ministry closed early (and had a very long delay at opening) and Untrainable Dragon broke down while we were watching (and we didn’t get to come back). A second day gives you a better chance to catch a ride/show if one of them is having a bad day.
C) Depending on what rides you are doing, I’m not sure the Express Pass is quite as useful (it may be different when you visit) at Epic. When I was there, three major rides (plus the Toothless meet) didn’t accept the EP (this might be different by the time of your visit, though), and I’m not sure the EP actually saves much if any time at the two shows. While the EP definitely saved us some time in line, it was less than 2 hours total that day. That might be because we didn’t have a chance to use it on as many rides (and the park eventually cleared out at the end, so there were very short waits for the 3 rides still open; Toothless had a long wait, still). I was very glad we had the pass since we only had one day, but I felt the EP was more useful overall in the other 2 parks.
D) And finally, I think one of the things that made us enjoy Epic despite missing rides and dealing with weather was the overall “feel” of the place. It was a lot of fun to walk around there in every one of the lands (in the other Universal parks, I think only the Harry Potter areas and the Seuss land are similarly well-done), and I’d vote for just plain having more time available in the Epic Park, even if not doing “more”.