I think it’s important to consider your major, especially since there is no guarantee that you will get into your major at UCSD. If you want to be in engineering, you should go to a school where you are able to study engineering. It doesn’t matter if UCSD is or is not more prestigious than UCI. If you don’t have the engineering degree, it is going to be much more difficult to convince employers that you are qualified for an engineering job.
Have you considered what you would do if you went to UCSD and did not get into the school of engineering? If you would not be happy at UCSD without the engineering major, then I would lean towards UCI.
Time-wise, there’s really not that significant of a time difference. You won’t be stranded if something happens at either place. It’s not nearly far enough that someone couldn’t come help you if there was a serious concern or issue. However, you’re an adult (or nearly an adult). You can handle being sick without your parents there to help you. Figuring out how to take care of yourself is part of growing up, but don’t worry, you would still be close enough that you could go home for a weekend or have help if you really needed it. I wouldn’t consider the distance to heavily.
The ponds aren’t that significantly different at UCSD and UCI that you would jump from being the big fish at UCI to a tiny fish at UCSD. I wouldn’t worry too much about that.
As for the campus, UCSD’s campus was built with walking in mind. You don’t typically have to walk from one edge of campus to the next. The lecture halls are on the edge of each college closest to the center, which cuts down the size of the campus that you would be walking around down tremendously. And besides, walking more is good for you =D UCI’s campus also isn’t exactly tiny, so you’d be walking around a fair amount there was well.
What are you concerned about in terms of campus life? I think both would be pretty comparable in terms of their opportunities to get involved.