should I keep him/her

Hi, I am after a little advice please! Last night my mom and I found a kitten wandering around late at night. The poor little kitty is obviously lost because he/she is well cared for but I think too young to be outdoors yet. I took kitty home to my flat and posted notices all around the area. I am hoping that a loving owner will be found. However, if not, one of my moms neighbours has said she will take kitty home. She already has 3 cats and lives in a house. The problem is this: when it was a choice between keeping a cat in a high rise flat with only a balcony (I would put up netting or mesh accross the balcony so kitty would get fresh air but not be able to get over it!) or the RSPCA, the choice was not so hard as myself and my 9 year old son have fallen madly, deeply in love with him/her (if I keep it I must get it sexed!!!), but now someone with a big house and garden have offered to take her, it just doesnt seem fair to keep a cat who must always stay indoors (other than balcony) in preference to letting her live where she can roam outside in the garden and round about. I will keep her anyway for a few days, until I am certain no-one will claim her, but I am really fretting about what I should do afterwards. Can anyone give me any helpful advice on which is REALLY the best option for this beautiful and loving little sweet kitten please? Thankyou very much and best wishes from Pauline