You can't go wrong either way, but I feel like you have answered your own question. If matching in California is a priority for you, then UWashington is the better choice. You are far more likely to match in CA from WA. It being the cheaper of the two schools, it's a no-brainer. Also, you'll understand this better once you're in clinicals, but true P/F during clerkships is nothing but a bonus when you're coming from an excellent school like UWashington. Not having to worry about honors, etc. frees you up to dedicate your clinical time to learning instead of kissing a**. Look at OHSU's public match list for the class of 2024 and you'll see we had no problems matching at excellent programs with a true P/F curriculum.
UMass is a great school, but so is UWashington. And Seattle is an awesome city, where you'll have all sorts of cool opportunities. It's expensive, but you'll have a good time.
Either way, congratulations. Whatever choice you make will be a good one.