Q:

When should you or shouldn't you use a specific fet? 

When are P channel mosfets used instead of N channel mosfets?  
When should you NOT use a P channel mosfet instead of a N channel mosfet, and vice versa? 
When should you use a Jfet? 
I just want to get a good comparison going between the different channel mosfets. 

A:

JFETs are rarely used (hard to find nowadays) but do have a place in high-speed analog circuits where they can be used as current sources or signal buffers. 
Otherwise, a similar question has been asked before: Switching DC with MOSFET: p-Channel or n-Channel; Low Side Load or High Side Load? 
I wrote a blog post about the different topologies.