The Beta58 is the upgrade of the SM58. Both great vocal microphones for singers. Price is about $100 for the SM and about $150 for the Beta version. They are bullet-proof and last for years and years.

The SM81C is quite sensitive but it isn't a shotgun. Senn 66's are about the hottest microphone I've come across. The 81 isn't that hot but it is still quite sensitive. I think it better fits the normal environment than the 66.

A Cardiod and a shotgun are two quite different microphones. I use both (although not the 66, I use an AT835B) in the appropriate situations. Sometimes both on the the same shoot. Each one plugged into a channel on the camera.

A shotgun is quite directional within its effective frequency range. Unfortunately, at lower frequencies, right where most of the noise in the world is generated, they are almost omni in characteristics. So they equally gather in target audio as well as garbage on the side.

Shure has an interesting document on video for sound at this URL:
http://www.shure.com/pdf/booklets/audio_for_video_production.pdf