I think Urban music has moved on and will probably leave Toni and Mariah behind... Whitney will probably be the most successful.
Judging by the 'success' of the respective singles of the 3 singles in the US, Whitney would have to be the most successful. Mariah's single even with the massive hype and afer being released for weeks to the radios, it has yet to reach the top 100 on the Billboard charts. It is only making an impact on the Adult Contempary charts, hence I can say that Mariah is not really impacting Urban music anymore. In fact, her record company has decided not to release the song as a single (the disc) in the US anymore.
Toni's new single is very urban, however it sounds like every other mediocre song out there, there is nothing that makes it special, without her signature vocals. Also, it is very different, very very different from anything she has done before, so her fans may just desert her... But if it is anything to go by, her music is rather up-to-date, maybe it is just bad production or choice of single. After listening to Whitney's new album, I think it is pretty good, more R&B than anything else. It has got good potential, and has some of her signature ballads in it. Though her single is not doing very well, I think that her it has potential and it a matter of time. It debutted at number 96 last week.
Unfortunately, Monica is not doing very well either...