Not sure what you base that opinion on!? Having witnessed many productions both sides of the pond there are just as many incredibly talented artists in both cities! What I can say is that having worked with and subsequently become good friends with a Broadway Valjean who swapped with our London Valjean for 6 months was that he preferred the more laid back approach to the Westend.
I think her talent may not seem so 'incredible' in NY where they are generally much more talented. Still it's a good break for her, hope the show is good.
As for Rachel, I believe her talent is especially unique and that Broadway will love her as much as we do over here. But the most important factor is that it has always been Rachel's dream to go to Broadway and so I for one am so delighted for her that her dream is becoming reality. Broadway appeals to many Westend performers as much as the Westend appeals to Broadway performers. They both have prestige for different reasons but ultimately a performer with such aspirations can understandably want to experience both.