The bluebird IS NOT application sensitive. The baby bottle has its own sound and is considered by many pro's to be superior for SOME applications.
The spark has some good reviews and some that claim it to be a mere toy. I saw it and it certainly did look like a toy. So much I didn't even bother trying it.
I have a bluebird and I seriously doubt it would be better.
The BB can be used on vocals, guitar cabs, overheads, acoustics and even mic'ing drums. A very good mic that everyone should have one of in the locker.