Invisalign Vs. Traditional Braces for Teens?

My daughter is interested in Invisalign, mainly because she is a sophomore in high school and our dentist where we live says she would need braces for 2 and a half years. She picked up a pamphlet for Invisalign and saw a case similiar to her bottom teeth (which are the main problem) and the time period was 7-11 months. Her top teeth are pretty much straight, but her bottoms are over-crowded. With two teeth behind the center ones. One on each side. She wants to straighten these but with the dentist saying it would be two and a half years with the wires on both top and bottom, so she is obviously not too keen with this. Would there be a difference between Invisalign and traditional braces for her? Also, the difference cost wise? Which would most likely be better for her?