There is a very good post on the community about this:
https://community.cisco.com/t5/intrusion-prevention-and/firepower-vs-ngips-vs-firesight-vs-firepower-management-center/td-p/2975375
However, your question is very broad and as a result, it is hard to answer. From an IPS perspective, both systems are utilizing Snort under the hood and TALOS for Threat Intelligence. The new Firepower hardware will provide better/higher throughput and better performance with TLS/SSL inspection due to dedicated crypto hardware. Thus, if you are planning on doing a pure NGIPS deployment, both systems will behave in similar fashion. On the other hand, if you are planning on running NGFW then things change since FTD (Firepower Threat Defense) combines the ASA and Sourcefire code/features.
I hope this helps! Let us know if you are looking for something more specific.
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