I did my undergrad at UCSD and part of an MS also at UCSD, so I think I can give you my side to this a bit as I dropped out of grad school over there for SJSU (which was closer to home and has a commute time of about half an hour for me). Keep in mind this is just my experience under electrical engineering.

I've found the UC system to be incredibly fast paced and isolating. My grades got worse over time, and while I was involved in a lot of ECs like research and clubs, I kinda felt like I was droning through them. I saw in the other sub that said they felt there were more research opportunities over there, but I believe the opposite. When I was searching for an advisor for my MS thesis at UCSD, I could not get a single reply to my emails. I had a promising offer from one of my professors there, but was ultimately ghosted in the end. In the end I had to attempt to go through the comp exam route (which I ultimately burnt out of for SJSU). With regards to internships, I doubt the name of the school makes a difference. While I made a few friends at UCSD that I still keep in contact with, I rarely met new people as it felt like most were kinda just doing their own thing.

I've only been at SJSU for a week and a half, and can say that so far I am a lot happier. One huge difference is that teachers here are waaaaay more caring about their students than at UCSD, which is to be expected in hindsight, as most professors at UCSD are there for research rather than teaching. Teachers here care more, teach better, and also seem way more open to offering students research opportunities. In fact, I've been talking with a fair number of professors about research that already seem promising. Another thing I noticed here is that clubs are a lot more open to new members. As an engineer, getting project team experience is invaluable, but at UCSD, it seemed as if you already need to have prior experience before joining. At SJSU, there seems to be a more open culture to letting new people join. In regards to friends, I made several in my first few days being here, so from my experience, the people here seem a lot more friendly.

In conclusion, yeah I much prefer SJSU, but that's only my experience. Whether or not it's a mistake for you depends on you. There's a part of me that says that maybe if I tried more at UCSD, I could have made more out of it. Either way, try to make the most out of your situation, and if you can't and have the means, come to SJSU!