I was thinking about this more. I think your question boils down to your classic lawyer answer.... it depends.
The physical skills required to play defense or offense? In that case it's offense. The physical skills on defense mostly happen in the weight room. A lot of the "skills" people talk about on defense like which hand to block with, never crossing your feet, boxing out etc. are actually kind of myths or are at least in the "it depends" category as well. Defense is all S&C and between the ears learned in practices.
Now if we take that question as "what is harder to learn?" it also depends on how you interpret the question.
I could give someone with motivation individual coaching a couple hours a week to refine their form, and then have them track their time and repetitions for the skills. With enough motivation you're going to get pretty darn good unless you're an absolute clutz. 20 minutes of dribbling drills a day, hundreds of shooting reps, and layups with both hands will get you pretty far. You still need team reps to learn the ins and outs of spacing and passing and such, but quite a few offense skills can be learned on your own with work ethic.
Defense isn't something you can learn on your own... ever. It's all feel for the game, heart, and having an insane IQ. I can't give you a plan to make 500 3s with good form a day, and voila, you're a shooter in year. You just have to play, play, play and get feedback from coaches and watch film. Couple all that work in practice with the mindset to murder someone on the court, and you get Steph Castle.
Learning passing on offense is much more like learning defense. It's all feel and repetitions. There's no shortcuts.