I never use Bixby Voice but I do use Bixby Home which I find more useful than Google Now since Google started pushing a bunch of trending news that have nothing to do with my searches and interests... given I have very little care for the Kardashians and other types crap, Bixby Home gives me the weather, my calendar, reminders, Facebook notifications and Twitter. I'd even like it if there were more apps I use compatible with it. I wanted a page that I could customize more than Google Now like OnePlus does with the shelves as well. Bixby Home gives me that.
The thing with Bixby Voice though is that I live in a French speaking area and my phone is mainly in French but Bixby is just available in English which would make a lot of my usage incompatible (SMS, tweets and so on). So Google Assistant is better for me in that respect (and I only use it to send SMSes while I'm driving.