You can use either ASA or Firepower Threat Defense (FTD) operating systems on your hardware platform:
ASA—The ASA is a traditional, advanced stateful firewall and VPN concentrator.
You may want to use the ASA if you do not need the advanced capabilities of the FTD, or if you need an ASA-only feature that is not yet available on the FTD. Cisco provides ASA-to-FTD migration tools to help you convert your ASA to an FTD if you start with ASA and later reimage to FTD.
FTD—FTD, also known as Firepower NGFW, is a next-generation firewall that combines an advanced stateful firewall, VPN concentrator, and next generation IPS. In other words, the FTD takes the best of ASA functionality and combines it with the best next-generation firewall and IPS functionality.
We recommend using the FTD over the ASA because it contains most of the major functionality of the ASA, plus additional next generation firewall and IPS functionality.
To reimage between the ASA and the FTD, see Reimage the Cisco ASA or Firepower Threat Defense Device.