I think depends on your situation. If you and your mother in law relationship is good then you can ask her to help you to take care of your baby. If not, better hire a helper. Cuz is difficult to ask your mother in law to understand what do you want. If you find something that you don't like may be you cannot tell her cuz she may get angry.
There are pros and cons on both. 工人 you have to pay $$ but you can tell her to do whatever you want, also some 工人 are not good so you'll worry about your child while you are not at home. 99 probably doesn't cost that much $$ but you cannot tell her what to do and you cannot correct the things she've done. She might have her own way of thinking but mostly they will treat your child really good (but becareful your child will be over spoilt). For my opinion, if $$ is not a big issue in your family, I will definitely choose 工人. I'll try to stay away from 99's as much as possible. The closer you gets with 99 the more arguements you'll have with her no matter how good your relationship is with her. Cuz you are really not her own borned daughter.