UCSD is no less competitive for premed than UW. There is no reputational or med-school-admissions-related reason to pay double for UCSD over UW. If you have other reasons to prefer UCSD over UW, or La Jolla over Seattle’s University District, and your family has the means to pay the difference for those reasons, then that’s a conversation you can have. But you wouldn’t be buying any meaningful advantage as a premed by paying more for UCSD. As a WA resident, you’ll have an advantage when applying to UW’s med school either way; and attending UCSD won’t confer an advantage at CA schools or anywhere else.
You’d get a strong education at either school, and the competitiveness for grades, etc. is probably about the same. I personally wouldn’t consider passing up the UW offer unless there were significant issues in terms of major/program (i.e. admitted to desired major at one school but not the other, or something like that) but you didn’t mention wanting to do CS or other difficult-to-enter major so I suspect there’s no such issue. As others have said, save the $ for med school and enjoy UW!