Talk about comparing apples to oranges!
You taking a detailed small dia. condensor mike, the SM81, and comparing it to a somewhat hyped large dia. condensor mike, the AT4040.
Both mikes represent completely different applications of use.
You REALLY should read the "sticky" in this forum to have an understanding of how mike type, dia. size, and pickup patterns relate to microphone application.
Now, no one's saying you can't break some rules, but the SM81 would not be MY choice for a vocal mike.
If you're seeking a vocal mike, get the 4040, if you want a detailed instrument mike, get the 81.