I've been both and am currently a SAHM. When I was working I would imagine how beautiful life would be if I could stay home and care for my family. Now in "Year 2" of staying at home I see how I really romanticized being a SAHM.
1. I would have time to do all the things I always wanted to do (learn a new language, play guitar, travel, sew, gardening, etc.)
I am not sure where my time goes, but I do know since I don't work I have little money. So, no classes at the community college. Had a yard sale and bought some flowers, so I am gardening right?
2. I would be able to take the kids to museums, trips, camps, etc.
See above (lack of $). I do my best but sometimes even gas money is tight.
3. I wouldn't have to do all my housework at 10 at night while exhausted after everyone else has gone to bed.
Trying to clean the house while kids and husband are awake is like shoveling snow in a snowstorm.
4. Weekends would be free because everything else would be taken care of during the week.
No explanation...just laughter! I REALLY believed that one!
5. My adopted son would be more secure and attached if I was with him full time.
Attachment is good, but as far as feeling secure...
True story from Target today: Turning down the juice aisle, oh, no, another cart and a man is pushing it. "MAN! MAN! MAN!" Then he decides since I laughed the man was OK. Proceed to the checkout..."MINE! NO! MINE!" Lots of tears. How dare that cashier scan our merchandise and put it in an evil bag! 
Usually I love being home and I am sure there are benefits to it that I won't realize until I am back to work, but some days I actually calculate how many days left until my son starts kindergarten. (I know how evil it sounds that's why I tell you and not people I actually have to see!) LOL! Now I have a better fantasy...being a rich SAHM!