Difference is the operating system that the chassis runs. ASA means that you will have the traditional ASA software running on top of the 2100 chassis. Apart from the initial hardware setup, everything will look exactly the same as the ASA5500 firewall from management (ASDM) and day to day operation. What you will NOT get with the ASA is the Next generation Firewall features like Application Visibility and Control, IPS, Advanced Malware Protection (AMP) and URL Filtering.
The NGFW version uses a new operating system called Firepower Threat Defense (FTD) that encompasses features from the traditional ASA plus the NGFW features mentioned above. This uses the Firepower Management Center (FMC) as a single point of management. If you are looking for these advanced features on the new Firewall, FTD is the way to go. The only word of caution would be that the FTD is fairly new, so there could be features that you use on the ASA that may not be supported/fully functional on the FTD. Also, there is no easy way to switch back between the 2 operating systems unless you completely re-image the hardware.
My suggestion would be to run your requirements by your Cisco SE/AM/partner and make sure that the FTD is capable of handling everything that you use today on the ASA. If yes, this would be the way to go forward.
