when to use NPT and when to use sweat

If you have threaded pipe, that would be one reason to use NPT
When I'm working with copper pipe, I prefer soldering. It's one less thing to go wrong. And, at my age, I prefer anything that will assemble without pipe wrenches.
I have to keep my wrists in shape for softball.

The "pipe" you are working with usually dictates what kind of flange, or connection, you use. However, if you are working with steel pipe, you would normally only use a threaded flange. If you have copper then you can either solder the flange on, or convert to an adapter and screw the flange on. In this case, it would depend on what kind of flange you have.