F5 AFM vs NG FW

It kind of depends what kind of Next-Gen FW you are going to be using and what you are trying to achieve. What's your business case? I'm working a lot with both F5 and Check Point. In most cases I would position the F5 ADC behind the Check Point, because the Check Point comes with superb management capabilities (detailed logging etc...).

You could also check out SSL Orchestrator to create a decrypted zone to put the Next-Gen FW into. This way you can save resources on your Next-Gen FW, becasue it will only need to inspect already decrypted traffic.