For someone like me who already took some Japanese classes back in college, 80/20 Japanese was perfect for me because I just wanted to focus on reviewing grammar. I actually learned a bunch of stuff that I either hadn’t encountered before in other resources, or was explained in such a way that things finally clicked for me.
In your case, if you’re just starting out with Japanese and are already liking Genki, then feel free to finish the Genki books first and flip through 80/20 Japanese later for anything that maybe didn’t cement the first time with Genki. If something is already working for ya, definitely keep up with that!