Talent isn't all you need to succeed though. Nobody with functional ears would say that Britney can out sing Christina, because she never could. However, Britney is the better artist. She has more star power. I think part of being a successful artist is making the kind of music that can appeal to a target audience. Britney did that WAY better than Christina did and had way more success in the earlier parts of her career than Christina. Christina was pretty much always in Britney's shadow.
Even now, Britney is getting paid more on her first season of X Factor than Christina is getting paid on her third season. Christina's last album didn't do as well as Britney's and I think the last time Christina hit #1 on the Billboard Hot 100 with a song as a solo artist was like what, over 10 years ago?
But yes, a good voice gets you nowhere these days. If it did, just about anyone coming off these reality singing competitions would be a major star, but the overwhelming majority of them just fizzle out.
Haven't heard Christina's new single but I hope her upcoming album is like Back to Basics. She also for some reason gets heavily compared to Lady GaGa these days, which I will never understand why.