I’m a bit of the reverse. Watched all of Weeds, but only ~2 seasons of Six Feet Under. I will unequivocally state that SFU is a better show. Better written, certainly. However it had a character I actively disliked, the ensemble nature bugged me just a bit because I was more invested in some stories than others and it just failed to hold my attention after the first season. I should give it another chance.
Weeds is what it is, a much weaker comedic (at times darkly comedic) version of Breaking Bad. Many, many people bailed after the first couple of seasons and while I don’t blame them, I mildly enjoyed the entire run. I probably wouldn’t watch the whole thing again but I don’t regret doing so. It was fine.
I’d recommend SFU as the stronger show, but they’re fairly different in tone. Weeds is quite a bit wackier and keeps trying to escalate the wackiness season by season in a game of self-upmanship.