I stayed at home with my son for the first year, but my husband changed jobs and I needed to earn some extra money. I decided to go and work at a church that had a preschool and "mothers day out" program. My son was in the same building as me and I could visit him whenever I needed. We both loved it--he could socialize and so could I. Six months later he was diagnosed with CF. I took two weeks off and was seriously considering quitting my job. Healthwise it probably would have been best to stay at home with him, but there are so many vital childhood milestones that come from socializing with other kids--young and old. My boss was awesome, she knew all about CF, and respected his needs. The other parents were understanding, as well. We stayed there until he started kindergarten and we loved it. I think watching your neighbors is a great opportunity, as long as you respect each other's needs. Make sure she understands the doctor visits (let her know well in advance of scheduled visits), if her kids are sick not to bring them, as well the unpredictablity of CF. The girls will play with each other and keep each other busy, you will have plenty of time to give Ashton the attention he needs. Go for it !
I completly agree with Melissa about the "mommy time". The first year of my son's life was spent home alone. I didn't have the money to go out and pamper myself, or take my son to the zoo, etc... I felt isolated and depressed. Once I was finally able to have some time to myself, I became a better mom. It is unhealthy to neglect yourself.
It is idealistic to say that the best thing for your child is to stay at home at whatever cost. A majority of parenthood is spent doing what works best for your family, not whats best for someone elses.