What is the difference between OTR and Signal protocols? The Signal Protocol is a relatively new secure messaging protocol that was recently implemented in Signal, WhatsApp, and several other messenging apps.  According to Wikipedia, the Signal protocol is an evolution of the OTR protocol (https://otr.cypherpunks.ca/otr-wpes.pdf).  

One of the major new features of Signal is that it allows for asynchronous messaging (i.e. both participants need not be online at the same time).

What are the other important differences between these protocols? Are there any instances in which OTR would be better suited than Signal?
Check out the blog post by Whisper Systems regarding the triple ratchet of OTR verses the double ratchet of Signal. Also, a prior article about synchronous versus asynchronous messaging is worth the read. Both of those should answer all of your questions.