She's slutting it up right now because she's trying to break into the same realm as Katy Perry/Rihanna/etc.. Breaking into the older market requires her to shed the Disney tween star image, which is why her tits are all over the place these days (I am not complaining). It's a little weird right now, watching concert videos of her singing her sweet tween pop songs to thousands of little girls while jumping around in incredibly sexy and revealing outfits. She's trying to straddle two markets and it usually doesn't work very well, but we'll see. I think this upcoming film she's in (which starts shooting this month, btw) has a much better chance of making her a star with adults than her music does. I really hope she succeeds, but it's fucking hard. No one else has ever done it.
Justice has a fair chance, I think, but the mere fact that she's with Nick is forcing her down a different path. Disney has a massive production umbrella that can foster Selena's career well into her adulthood with star vehicle shit like Monte Carlo. That kind of crap is what killed Hillary Duff and I hope Selena avoids it in the future. Justice, on the other hand, is going to have to make the jump on her own because Nick's power ends when Victorious does. From what I can tell so far, she's done a really good job of making the leap, picking and winning roles in good films. I have absolutely no idea if her music will get her anywhere (I doubt it. It'll probably land about the same place Cosgrove's music has) but she's handling her acting career the right way.