However, upon listening to the song, I just feel that there's something "off" about it -- and it's mostly Britney's voice.
The original song, carried by Rihanna's vocals, is a force of nature, powerful vocals over driving beats. The imagery evoked by the overall tone of the song perfectly matches the lyrical content, and Rihanna is the engine that powers the whole thing. On the remix, Britney's vocals work well enough with the toned-down instrumentals over her verse, but they don't fit at all with the chorus, and it's painfully obvious. For all the maturation in her life outside of music, vocally she still sounds like a young girl "trying on" Rihanna's edgier style.
The remix really shows the fundamental difference between the two artist: while Rihanna is obviously a good dancer, entertainer and sex symbol, her voice is still her most powerful asset. Britney's voice for all it's positive qualities, isn't "powerful", and she ends up sounding overwhelmed on this track.