What are the differentiators to look out when choosing a CRM?

As with any project or vendor selection, it's important to focus on your requirements and ensure the solutions you're looking at meet those. 

For me and my company integration is a major part of CRM. If you look at all the CRM systems they are very similar in the functionality they provide and how you differentiate from others (and customize)  is by having integration available to you aka Zapier and API access. 

Cost is always one, but having just done this ourselves, I think you need to take a step back and look at what the true goals/use case for the CRM will be.  Integrations with some of your other tools/platforms should be a consideration as well.  

Thorough research, evaluations and POCs if possible are critical in selecting the right CRM for your company.  The big names out there are top of the line, but there may be a vendor out there you've never heard of that is designed specifically for your industry.  

Integration capabilities is a big factor, along with your vendor having a capable and well staffed support group should you need help.