DD did not attend the admit day for the school she would eventually attend because it was the same day as another school’s admit day 400 miles away. We book ended her spring break with admitted student visits two schools she was considering but hadn’t visited yet. By the end of weekend two, she knew would be attending neither of them. It would have been so much fun to go to the one for her chosen school, but it was really important to eliminate her other options, and this was the only way to do it. Note that we had toured her chosen school earlier in the year, so she wasn’t going in blind.
As for summer orientation (3 days, 2 nights, as I recall), it was mandatory (they registered for classes for the fall, among other things), but I’m sure she would have attended anyway.
OP, if you must pick between the 2, I’d say do the orientation. You will get a lot more out of it.