Okay lets see... I need a moment to figure my standings here.
We have Wonder Girl and Jubilee. Both started out as low level heroes, who quickly rose the ranks to become two of the most popular teen female heroines for DC and Marvel.
Wonder Girl of course has super-strength, flight, bullet proof braclets and a lasso of lightning. She once led Young Justice, and has shown time and again to be worthy of the title Wonder Girl. She's been trained by the Amazons of the Myth, who are some of the greatest warriors in the galaxy. She's one of the best of the Teen Titans and has a grand future before her.
Then there is Jubilation Lee. A creative, agile 17 year old mutant with the ability to create plasma blasts that can go from simple fireworks to full out energy blasts that could destroy an entire city block. She too has led a team of teenaged heroes for a short while (during the last year of Generation X she became the field leader). She has worked with Wolverine and the X-Men (who trained her in martial arts, tracking skills, and different forms of combat) on mutliple occassions and during that time has proven herself to have a suprisingly skilled analytical mind such as the time she tracked down her parent's killer using detective skills. She has also been doing the super-hero job longer than Cassie.
Frankly both of these kids are strong characters.
Though in the end, I vote for Wonder Girl. Not because I think Jubilee isn't skilled enough to hold her own, but Wonder Girl's powers are more versitial.