Translation:She has a cute cell phone.
女の子 is pronounced "onnanoko" and 少女 is pronounced "shoujo". From what I could find, 女の子 is used mostly for preteens and 少女 for teenagers or slightly before ("tweens"). There are some people who call teenagers 女の子, but it might be seen as a little weird? This was just what I gathered; I could definitely be wrong
Both could work, actually, but it would change the importance of the sentence to the having of the cellphone instead of the girl if you used が. Most of the time, they're pretty interchangeable, but they really serve two very different purposes which are hard to explain in English.