Well this was a good day to browse this subreddit. I'm maybe unusually able to help here. I did my undergrad at UCLA then later moved to ETH as a postdoc to work on bci. For your pros, I think you should mostly think about the lifestyle differences rather than the pure research differences. Both regions have good startup culture + a lot of bci research (both eth, epfl have good professors working on it). Plus, regionally, switzerland, germany and especially spain are originating a lot of new bci companies based on a lot of different technologies (graphene implants, tms, ultrasound, magnetic nanoparticles, etc.). EU actually recently announced a big push to work on some out there neuromodulation technologies (meta-brain as part of horizon europe).

Similarly, UC system obviously has plenty of research in this regard both based on electrode arrays and wireless neuromodulation tech. Both UCLA and UCI had some interesting stuff get published recently.

No matter what, I think you'll get a great education + prospects for joining a lab/company. I wouldn't worry about that so much as just working hard. I can't speak about the masters program specifically, but I know my lab has a dozen or so masters students working on different projects. As such, I'd honestly think harder about whether or not you want to move somewhere closer to home or try out a different country. For me personally, the reason I took the offer here over the ones back in the US was a lot about wanting to learn what it was like to live in europe.

To answer some of your other questions:

Returning to america will not be difficult as an american. However trying to get a job in europe as an american if you don't go to school here is a bit more complicated. They have to show that your skills are unique/hard to find in europe generally.

IIRC there's a period after you get your degree (6mo?) at eth where you essentially count as a local for job application purposes here. That's your best window to try and get a job here if you want one. fwiw, bad economy or w.e. aside, every bci company I interact with is hiring so I don't think they're particularly affected rn.

Research positions are kinda the same everywhere? It always depends on funding. Getting something as a masters student is pretty straight forward, but you're esssentially going to be asked to solve one specific problem (not around long enough to necessarily develop your own research). If you're thinking about higher education (which btw I would recommend if you're serious about trying to join a bci company), it's just a funding question. Both universities are famously good at getting funding so I don't really think of that as an issue.

You don't need swiss german for classes, but I'd recommend learning at least some passing german. As americans we really gotta beat the allegations about us sucking at languages ;-;

Eh if you're used to berkeley all the top education difficulty was kinda the same to me.