Both schools are great institutions and I think
@CyrilFiggis gave you an awesome rundown. I was accepted to both schools for undergrad, but went the USC route because as an OOS student I had awards making COA equivalent.
USC - lack of impaction, solid pre-med advising (although they don't advertise this amenity well to transfers, so make sure to seek it out), better athletics (not biased at all), incredible network, terrible area, heavier cost for most
UCLA - heavy impaction leading to lack of course availability and extended undergraduate timeline, fierce competition, don't know much about advising here, athletics are okay I guess, incredibly nice area, significantly cheaper tuition for instate students (esp. with your scholarship situation)
Keck's campus is not connected to the undergrad campus at USC as it is at UCLA. There is a bus that runs between these places, but the interchange area with the junction of the 110 and the 10 and the 10 and the 5 is always jammed, making this ~5 miles commute half an hour or more which can lead to sticky scheduling. There are a multitude of hospitals, school associated or otherwise, near either location where you can knock out volunteering/shadowing hours.
Both institutions are known for producing tons of research and you likely won't have a problem finding somewhere to get involved at either institution. The difficulty of this and volunteering will fall much more on your character and persistence than institutional availability.
Essentially, neither school is going to be 'easy', so work hard to maintain your numbers and to network with professors for LORs. In the end, UCLA probably makes more sense for you with your significant cost differential. For a future medical student, going significantly into debt here when choosing between such similar ranking institutions is setting yourself up for more hardship decades down the road than is necessary. UG institution really doesn't make much of a difference in the grand scheme of selection factors, especially not between these schools.
That being said, I think the prototypical premed student locked in the library 16 hours a day everyday would be more common at the latter than the former; SC students tend to balance a healthy social life with academic success quite well while I find this is more the exception than the rule at UCLA (no offense to any UCLA students, just walk around Powell to see what I am talking about).
Also agree with
@Lucca for the majority of his post, but his numbers are misleading. UCLA produces the most medical school
applicants in the country (961, which is 140 more than the next highest school and represents approximately 2% of all applicants in the country), not medical matriculates. The state of CA in general sucks for applying to med school statistically, with 37% (2438/6520) gaining acceptance anywhere. I don't think it can be said if either school has more success with eventual medical school matriculation without further analysis at the institutional level, but that number of premeds should give you insight to the type of competition you should expect for grades at UCLA.
