I've been to the Universal parks 50+ times, and here's what I would do:
If it were me and I had never been before, I would choose Studios and Islands. Velocicoaster, Hagrid's, Hulk, Spiderman, Forbidden Journey are all super fun rides that you should definitely do. Over at Studios, Mummy and Gringotts are my faves.
Sure, Epic definitely has some great rides and attractions and some of the best in-land theming I've ever seen, but it is REALLY hard to get everything done in one day. We chose to go 2 days so we didn't necessarily have to feel rushed to get everything done in one day, and we were even rushing then to get everything done. With breakdowns, weather, and long lines it is really hard to get everything done. If you can afford an Express, it would definitely be worth it and would let you get everything done much faster and you could get everything done for sure.
That doesn't mean it can't be done though. I had a friend go about a week ago and did most things, while skipping some others. If I was going to skip rides, I would skip Curse, Yoshi, Dragon's Racers Rally, Constellation Carousel, and possibly Fyre Drill if you didn't care to get wet. The HTTYD show is much better than the HP one in my opinion, and I think you could skip the HP one. I would recommend the HTTYD show if you have time and it's not broken (I had a rough experience with this show. Tried 3 different times over 2 days to do it before it finally opened). I did not see the HP show fully, as it broke down right before it started. I did get free Express though! Sadly no Ministry with the Express card we got.
The reason I would choose Studios and Islands though is simply because there are more rides and more big hitters there. You mentioned wanting to get on Velocicoaster, which is an absolute amazing ride. I just feel like you would get more out of Studios and Islands than you would from Epic. Epic might even be frustrating if you can't get everything done you wanted to do. Not sure what the crowd trends have been lately, but the best day to go to Epic was surprising the weekend. As of right now (7:50) the wait times are actually pretty good right now (everything under and hour with Monsters and Fyre Drill on delay).
All that said, if it were me I would choose Studios and Islands over Epic, but Epic is a great park too. You didn't ask for all of that, but maybe it helped. I love talking all things Universal! Let me know if you have any questions, I'll probably have an answer