In my opinion, you should keep her with the babysitter you're with! Does she go out to the park, library, etc. to interact with other children? I am a SAHM so my two never went to daycare and they did just fine when they went to Kindergarten - I had gotten them involved with kids by going to the park, library programs, etc. Even Barnes & Noble book store had a little reading time they loved going to.
As for learning new things - I was always "teaching" my kids new things and they didn't even realize they were learning! We would play games and do all kinds of stuff. They would "help" me with everything; and no question ever went unanswered. When they got to Kindergarten, they were always right on track, if not ahead, of the other kids.
Oh, and now...they're both Straight A Honors High School Students!! Needless to say, I am VERY proud of them - especially because THEY push themselves and WANT to succeed; I just got them started!!!
Good luck!!