I've heard 3 tracks, the Metronomy one (I absolutely love it, but as I said it is an album track and not a single. Very weird in a very good way. - It's very Sophie at the same time.), one of the Armin van Buuren tracks (AMAZING - an uptempo hands-in-the-air stomper) and a mid-tempo / ballad (don't know who produced it or its name, it was nice but it didn't blow me away, but again I only heard it once). Armin's track was incredible. I still (after almost 3 months) sing the chorus in my head and I'm still bedazzled at it and shocked it wasn't the first single. Bittersweet better be a masterpiece...
Sophie's decision to have 2 (actually 3) singles before the album sparked a tiny bit of unwanted frustration on the PopJustice forums (read here). So far, we know the song will hit the radio early next month, Sophie will be filming the video soon and we will have the release of the single in April. The plan hasn't changed, there is no need to fuss.
To confirm the quote above about Metronomy, an insider also said that the Metronomy track is very weird, a lot like their material. Somehow i never liked any of the Metronomy tracks that i have heard. I hope "Make A Scene" turns out to be amazing.
Right now, keep your eyes peeled for Can't Fight This Feeling. Bittersweet should be on the radio soon. C'mon Soph!