This is my opinion. I’m sure others will feel differently. Academically, the two schools are very similar. Both have strong Bio Sci departments. Both schools offer resources and advising for a variety of healthcare fields (medicine, pharmacy, dentistry, optometry, PT, OT, etc.). I feel that is important.
Socially, I think Davis wins out. Based on my children’s friends at both schools, STEM students at Davis have more of a mindset of “we’re all in this together.” You’ll see more kids forming study groups, sharing notes and working on projects together. The professors encourage it. Irvine STEM students tend to be a little more solitary. Personally, I hear UCI students referencing finding out about jobs/internships/opportunities through the student-run pre-professional clubs. I don’t hear many UCI students talking about pre-health advising. That could just be the students that I encounter.
Thanks for reminding me that I posted about Davis on the UCSB waitlist thread. I’m going to bring some of that post over here for others who might be interested
Regarding advising, I feel that the pre-health advising at UC Davis is better than UCSB. That said, advising only helps if a student takes advantage of it.
Davis pre-health advising offers a pre-health conference , med-school boot camp , application and interview prep , a MCAT mock exam and much more. Davis also has a health sciences LLC in the dorms (note: more students apply for it than there is space).
UC Davis Pre-Health Advising https://hpa.ucdavis.edu/
UCI Pre-Health Advising https://undergraduate.bio.uci.edu/pre-health-professional-advisement/
I poked around a bit on the UCI pre-health advising. They have a couple of videos that explain where students can go for resources. The videos are a few years old and still reference the campus being shut down for COVID. They have links to a lot of documents where students can read about different health careers. They don’t seem to have many live, in-person events. I looked at the list of “Clinical & Community Volunteer Opportunities for Pre-Health Students” and found that it hadn’t been updated since 2018. 
That might explain why UCI students use their student-run clubs as a resource rather than advising.
Regarding visiting this Sunday, have you looked at this thread with Davis suggestions? Are you a member of the Davis Parent FB group? I’ve seen current parents and students volunteer to meet with prospective students and show them around. The UCI parent group won’t let parents join until their student commits.