I actually went to both schools (Wash U undergrad, Cal for grad), not in your major though (physics/German at Wash U and then physics at Cal). I can’t speak to the actual department. Everyone else has covered the differences pretty well… both are great schools with different strengths. Wash U will give you smaller class sizes for the most part, a more ‘upscale’ campus feel which you may or may not prefer, a more suburban experience. At Berkeley you’ll have much better weather, more international recognition and diversity, access to one of the most beautiful parts of the world (okay, that’s subjective, I still live in the Bay Area and am biased ;-)). I felt like there was more hand-holding at Wash U although that might have been because I was an undergrad rather than a grad student there. There was definitely more nickel-and-diming at Cal (extra charges for photocopying, that sort of thing whereas at Wash U once your parents paid tuition everything else was included. This was over 30 years ago though and in the grand scheme of things probably shouldn’t be what you base your decision on).
In the end they are both great schools.