I'd say they're both hard, but I've not done either (I'm pregnant). My mother did both, and working was harder, because she still had to raise kids, clean the house, cook, do laundry, help with homework, etc, on top of going to work. I'd say because of that, working is usually harder, because you do what a SAHM does, plus go to work.
Depends entirely on the person and their situation. For some working will be a cinch and SAH extremely difficult, while for others the opposite will be true. Either is going to be hard if your SO and/or kids refuse to help out and only make more work for you to deal with. It's all relative to what each person can handle.
I think it depends on the person. For me staying at home was a breeze. I don't mind cleaning, cooking and teaching my children. I didn't have deadlines, endless meetings and a 80 clients depending on me. I just had my 4 children to depend on me.
I've done both and they are both hard and easy. Now, I think the ideal way is working the way I use to. I left after my boys left for school, was home before they got home, had weekends and holidays off and still had a day or two off during the week if I wanted. Best of both worlds!