I was looking at the Shure SM7B, trying to figure out mic sensitivity and why I would buy one that requires an auxiliary preamp, when I can buy one with higher sensitivity and connect it directly to my interface.  Any info would be appreciated.

A lot of people use the Cloudlifter to raise the output level:
 
https://www.sweetwater.com/store/detail/SM7BCloud--shure-sm7b-with-cloudlifter-cl-1
 
This should allow you to connect it directly to your interface.

Thanks, but I'm wondering why I would buy the Cloudlifter when I could just buy a different mic with higher sensitivity.

You buy a mic for its sound not how sensitive it is. Like most dynamic mics they need a good boost.

If your mic preamp provides enough (clean) gain, you won't need an auxiliary preamp like the CloudLifter.
I use an SM7b with a Neve Portico-II... with zero issues.
 
The SM7b is often used in scenarios where you don't want to pickup much of the room.
The SM7b was used on the vocals for Thriller.
 
SM7b is also good on guitar/bass cab, Voice-Over, etc.
 
Yes, you can get a more sensitive microphone.  
If you're mic'ing acoustic guitar in a good sounding room, the SM7b is not what you want.
If you're recording a rock vocal in a room with less than ideal acoustics (or higher ambient noise), an SM7b is a great choice.
 
 

Thanks for the replies.  It looks like I'm not going to "gain" much by switching to a dynamic mic for vocals, will probably just stay with condensers.