Spoiler Warning for Project Hail Mary and The Martian (probably, but the book's been out for 12 years)
Saying that these two books/stories are opposites isn't even a joke anymore, it's just an accurate description
Yeah, the stuff about Earth coming together to save one man in space vs. One man in space trying to save Earth.
But also all the space program people going on almost no sleep trying to figure out ways to send supplies to Mark?
Rocky would never allow that.
Grace destroying his circadian rhythm almost destroyed the mission. "Humans stupid without sleep". Wise words, Rocky.
I also realized that Ryland and Mark end up with opposite problems.
Watney has a ton of vitamins, the issue was him running out of calories.
Grace survived on Erid because the Taumoeba gave him calories, but he got no nutrients from that. The Eridians had to synthesize the necessary vitamins and minerals.
If there was ever a time to trade lunches, that'd be it.
This one isn't really opposites, but more of a funny parallel. Ryland spending weeks making chain links compared to Mark spending weeks in the rover. And at the end of it, they’re both just really sick of these things.
Also Rocky being there to care for Grace vs. Watney constantly endangering himself (mostly with fire).
Bonus: I saw a post that said Ryland should just take off running when he first gets his biodome, and so I just kept picturing that when I got to the scene of Watney returning to the Hab, and he just ran around like crazy.
Edit: Another opposite trait is Watney saying he's not that great at naming things ("pirate-ninja" being a term he randomly assigned for energy consumption) vs Grace coming up with multiple cool and comprehensive terms (naming "Astrophage", "Eridians", and "Taumoeba"; for a few examples).