From your post I get the sense that there isn't a special situation (apart from cost) that makes one the universities more attractive academically and socially to you -- as for example, a special program or professor or friend, or an extracurricular activity/sport etc.
The 2 universities are both excellent, and they are similarly ranked. Prestige wise I don't think that there's a significant difference between the 2. If anything University of Washington as the flagship of the Washington State system is probably better known outside of California.
The ambiance and the weather are the likely compelling differences. University of Washington is close to exceptional hiking/climbing/sailing/skiing, whereas UCSD has its own attractions -- i.e. beach, desert. The winter weather in Seattle is so-so, whereas the winter weather in San Diego is exceptionally fine.
My opinion is that unless there are special connections that have appeal, or the weather (especially in winter) is a driver for you, that the University of Washington is the better choice given the $10000 per year difference.