I would go with the Brother XL3750 which is $119
88 on line at Wal-Mart or the Brother CE5500PRW $139.00 or CE5000 $149.00 both at Wal-Mart also.
You said your niece is 12...I have a 14 year old daughter and 15 year old niece. My daughter is not into sewing but my niece is a little. I know what they gravitate to when we look at machines in the stores. I agree to some extent with the older machines. They are more sturdy and usually more reliable and would be great to learn on. But, she is a 12 year old. They usually like buttons and fancy stitches. Also, any of these 3 machines are lightweight.
Unless you know she will definitely be leaving it in one place, she will probably want to take it from your house to her house. She may even want to take it with her to a sleepover with her girlfriends and have a fashion party. That's what my daughter and niece used to do. They had great fun when they were younger making dresses, pocketbooks and hats then modeling them for me and my husband.
You may also want to think of what you might want for a back up machine incase she decides sewing/quilting is not what she thought it was.