I would guess that the path to med school is about equally steep at both schools. UCSD’s med school acceptance rate is significantly lower than UCLA’s. So perhaps UCLA weeds out more premeds, earlier in the process. But - just my opinion - if I were going to get weeded out, I’d rather it be earlier, so that I could devote the rest of my time as an undergraduate to making progress on an alternative path, than at the point of med school decisions when more than 60% of med school applicants coming out of UCSD get rejected.
The premeds I know at UCLA report that many of their required premed classes have lots of repeating students in them, trying to raise their grades from the first attempt… which makes it all the harder for the first-timers to do well. Premed at any UC is tough in this way. But I don’t think you should be afraid of measuring up at UCLA in particular. I certainly wouldn’t go 10K/year out of pocket for UCSD over UCLA! You’re a great student who is just as capable as the rest; use your resources to be PROACTIVE - set up tutoring from day one, form study groups, stay ahead and never ever wait for the first bad grade to put supports in place. Make your humility and trepidation work for you by utilizing every possible support from the beginning and you will leave those who are overconfident in the dust. You absolutely could thrive at UCLA, so weigh it on the attributes that matter to you, not through a lens of intimidation or self-doubt.
That said, if you really prefer UCSD, call up Financial Aid and ask what’s up. There shouldn’t be that big a difference in the aid formula!
Congrats and good luck!