Apples and oranges. I personally don't feel being a mom can be compared to a job.
Doesn't mean it's not the hardest thing you will ever do. I don't look at being a mom as a "job". I look at it as the most amazing and challenging blessing I've ever had. Sometimes it's easier caring for my son than being at work, sometimes it's not. It's always changing.
At the moment, things are incredibly difficult for me all around, at work and home. Maybe next week will be worse or better, IDK.
I will say the pressure of screwing up as a parent vs screwing up at my job is fair more terrifying. As for what's physically more challenging ? Depends on the day, week, month. Right now working over 40 hours a week with sciatica at 33 weeks along w/ a one year old is FREAKIN HARD. It's hard at work, it's hard at home, it's just always hard. There will be easier times and more difficult times than this I imagine.