Headset mics are great when you are constantly moving around, like a drummer. but you will always sacrifice a little for the benefit of movement.

Allot of singers will use the proximity effect with microphones, basically pulling away or getting real close, to help in control volume, especially if your really need to belt something out without blowing away everyone.

Also of you need to talk to someone, sneeze, cough, suck up some phlegm, you can add a cough switch but that's one more thing.

Also, allot of headset mics tend to be more sensitive and you pick up allot of breathing, since its so close to your mouth.

Crown, Senn, Shure, all are great brands, but also have different levels of each type of mic they make.

As for the frequency, when someone buys a wireless mic from me, I will call for example Shure, and they have frequency charts for the US, I tell them where they will mainly play , and they can give me the best freq with the least amount of interference for that area